arm64: Don't call NULL in do_compat_alignment_fixup()

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

arm64: Don't call NULL in do_compat_alignment_fixup()

do_alignment_t32_to_handler() only fixes up alignment faults for
specific instructions; it returns NULL otherwise (e.g. LDREX). When
that's the case, signal to the caller that it needs to proceed with the
regular alignment fault handling (i.e. SIGBUS). Without this patch, the
kernel panics:

Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
Mem abort info:
ESR = 0x0000000086000006
EC = 0x21: IABT (current EL), IL = 32 bits
SET = 0, FnV = 0
EA = 0, S1PTW = 0
FSC = 0x06: level 2 translation fault
user pgtable: 4k pages, 48-bit VAs, pgdp=00000800164aa000
[0000000000000000] pgd=0800081fdbd22003, p4d=0800081fdbd22003, pud=08000815d51c6003, pmd=0000000000000000
Internal error: Oops: 0000000086000006 [#1] SMP
Modules linked in: cfg80211 rfkill xt_nat xt_tcpudp xt_conntrack nft_chain_nat xt_MASQUERADE nf_nat nf_conntrack_netlink nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 xfrm_user xfrm_algo xt_addrtype nft_compat br_netfilter veth nvme_fa>
libcrc32c crc32c_generic raid0 multipath linear dm_mod dax raid1 md_mod xhci_pci nvme xhci_hcd nvme_core t10_pi usbcore igb crc64_rocksoft crc64 crc_t10dif crct10dif_generic crct10dif_ce crct10dif_common usb_common i2c_algo_bit i2c>
CPU: 2 PID: 3932954 Comm: WPEWebProcess Not tainted 6.1.0-31-arm64 #1 Debian 6.1.128-1
Hardware name: GIGABYTE MP32-AR1-00/MP32-AR1-00, BIOS F18v (SCP: 1.08.20211002) 12/01/2021
pstate: 80400009 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : 0x0
lr : do_compat_alignment_fixup+0xd8/0x3dc
sp : ffff80000f973dd0
x29: ffff80000f973dd0 x28: ffff081b42526180 x27: 0000000000000000
x26: 0000000000000000 x25: 0000000000000000 x24: 0000000000000000
x23: 0000000000000004 x22: 0000000000000000 x21: 0000000000000001
x20: 00000000e8551f00 x19: ffff80000f973eb0 x18: 0000000000000000
x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000000
x14: 0000000000000000 x13: 0000000000000000 x12: 0000000000000000
x11: 0000000000000000 x10: 0000000000000000 x9 : ffffaebc949bc488
x8 : 0000000000000000 x7 : 0000000000000000 x6 : 0000000000000000
x5 : 0000000000400000 x4 : 0000fffffffffffe x3 : 0000000000000000
x2 : ffff80000f973eb0 x1 : 00000000e8551f00 x0 : 0000000000000001
Call trace:
0x0
do_alignment_fault+0x40/0x50
do_mem_abort+0x4c/0xa0
el0_da+0x48/0xf0
el0t_32_sync_handler+0x110/0x140
el0t_32_sync+0x190/0x194
Code: bad PC value
---[ end trace 0000000000000000 ]---