bpf: Fix null pointer dereference in resolve_prog_type() for BPF_PROG_TYPE_EXT

In the Linux kernel, the following vulnerability has been resolved:

bpf: Fix null pointer dereference in resolve_prog_type() for BPF_PROG_TYPE_EXT

When loading a EXT program without specifying `attr->attach_prog_fd`,
the `prog->aux->dst_prog` will be null. At this time, calling
resolve_prog_type() anywhere will result in a null pointer dereference.

Example stack trace:

[ 8.107863] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000004
[ 8.108262] Mem abort info:
[ 8.108384] ESR = 0x0000000096000004
[ 8.108547] EC = 0x25: DABT (current EL), IL = 32 bits
[ 8.108722] SET = 0, FnV = 0
[ 8.108827] EA = 0, S1PTW = 0
[ 8.108939] FSC = 0x04: level 0 translation fault
[ 8.109102] Data abort info:
[ 8.109203] ISV = 0, ISS = 0x00000004, ISS2 = 0x00000000
[ 8.109399] CM = 0, WnR = 0, TnD = 0, TagAccess = 0
[ 8.109614] GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
[ 8.109836] user pgtable: 4k pages, 48-bit VAs, pgdp=0000000101354000
[ 8.110011] [0000000000000004] pgd=0000000000000000, p4d=0000000000000000
[ 8.112624] Internal error: Oops: 0000000096000004 [#1] PREEMPT SMP
[ 8.112783] Modules linked in:
[ 8.113120] CPU: 0 PID: 99 Comm: may_access_dire Not tainted 6.10.0-rc3-next-20240613-dirty #1
[ 8.113230] Hardware name: linux,dummy-virt (DT)
[ 8.113390] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 8.113429] pc : may_access_direct_pkt_data+0x24/0xa0
[ 8.113746] lr : add_subprog_and_kfunc+0x634/0x8e8
[ 8.113798] sp : ffff80008283b9f0
[ 8.113813] x29: ffff80008283b9f0 x28: ffff800082795048 x27: 0000000000000001
[ 8.113881] x26: ffff0000c0bb2600 x25: 0000000000000000 x24: 0000000000000000
[ 8.113897] x23: ffff0000c1134000 x22: 000000000001864f x21: ffff0000c1138000
[ 8.113912] x20: 0000000000000001 x19: ffff0000c12b8000 x18: ffffffffffffffff
[ 8.113929] x17: 0000000000000000 x16: 0000000000000000 x15: 0720072007200720
[ 8.113944] x14: 0720072007200720 x13: 0720072007200720 x12: 0720072007200720
[ 8.113958] x11: 0720072007200720 x10: 0000000000f9fca4 x9 : ffff80008021f4e4
[ 8.113991] x8 : 0101010101010101 x7 : 746f72705f6d656d x6 : 000000001e0e0f5f
[ 8.114006] x5 : 000000000001864f x4 : ffff0000c12b8000 x3 : 000000000000001c
[ 8.114020] x2 : 0000000000000002 x1 : 0000000000000000 x0 : 0000000000000000
[ 8.114126] Call trace:
[ 8.114159] may_access_direct_pkt_data+0x24/0xa0
[ 8.114202] bpf_check+0x3bc/0x28c0
[ 8.114214] bpf_prog_load+0x658/0xa58
[ 8.114227] __sys_bpf+0xc50/0x2250
[ 8.114240] __arm64_sys_bpf+0x28/0x40
[ 8.114254] invoke_syscall.constprop.0+0x54/0xf0
[ 8.114273] do_el0_svc+0x4c/0xd8
[ 8.114289] el0_svc+0x3c/0x140
[ 8.114305] el0t_64_sync_handler+0x134/0x150
[ 8.114331] el0t_64_sync+0x168/0x170
[ 8.114477] Code: 7100707f 54000081 f9401c00 f9403800 (b9400403)
[ 8.118672] ---[ end trace 0000000000000000 ]---

One way to fix it is by forcing `attach_prog_fd` non-empty when
bpf_prog_load(). But this will lead to `libbpf_probe_bpf_prog_type`
API broken which use verifier log to probe prog type and will log
nothing if we reject invalid EXT prog before bpf_check().

Another way is by adding null check in resolve_prog_type().

The issue was introduced by commit 4a9c7bbe2ed4 ("bpf: Resolve to
prog->aux->dst_prog->type only for BPF_PROG_TYPE_EXT") which wanted
to correct type resolution for BPF_PROG_TYPE_TRACING programs. Before
that, the type resolution of BPF_PROG_TYPE_EXT prog actually follows
the logic below:

prog->aux->dst_prog ? prog->aux->dst_prog->type : prog->type;

It implies that when EXT program is not yet attached to `dst_prog`,
the prog type should be EXT itself. This code worked fine in the past.
So just keep using it.

Fix this by returning `prog->type` for BPF_PROG_TYPE_EXT if `dst_prog`
is not present in resolve_prog_type().