bpf: Fix NULL pointer dereference in bpf_sk_storage_clone and diag paths

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

bpf: Fix NULL pointer dereference in bpf_sk_storage_clone and diag paths

bpf_selem_unlink_nofail() sets SDATA(selem)->smap to NULL before
removing the selem from the storage hlist. A concurrent RCU reader in
bpf_sk_storage_clone() can observe the selem still on the list with
smap already NULL, causing a NULL pointer dereference.

general protection fault, probably for non-canonical address 0xdffffc000000000a:
KASAN: null-ptr-deref in range [0x0000000000000050-0x0000000000000057]
RIP: 0010:bpf_sk_storage_clone+0x1cd/0xaa0 net/core/bpf_sk_storage.c:174
Call Trace:
<IRQ>
sk_clone+0xfed/0x1980 net/core/sock.c:2591
inet_csk_clone_lock+0x30/0x760 net/ipv4/inet_connection_sock.c:1222
tcp_create_openreq_child+0x35/0x2680 net/ipv4/tcp_minisocks.c:571
tcp_v4_syn_recv_sock+0x123/0xf90 net/ipv4/tcp_ipv4.c:1729
tcp_check_req+0x8e1/0x2580 include/net/tcp.h:855
tcp_v4_rcv+0x1845/0x3b80 net/ipv4/tcp_ipv4.c:2347

Add a NULL check for smap in bpf_sk_storage_clone().

bpf_sk_storage_diag_put_all() has the same issue. Add a NULL check
and pass the validated smap directly to diag_get(), which is refactored
to take smap as a parameter instead of reading it internally.

bpf_sk_storage_diag_put() uses diag->maps[i] which is always valid
under its refcount, so diag->maps[i] is passed directly to diag_get().