bpf: Convert bpf_selem_unlink_map to failable

To prepare for changing bpf_local_storage_map_bucket::lock to rqspinlock,
convert bpf_selem_unlink_map() to failable. It still always succeeds and
returns 0 for now.

Since some operations updating local storage cannot fail in the middle,
open-code bpf_selem_unlink_map() to take the b->lock before the
operation. There are two such locations:

- bpf_local_storage_alloc()

  The first selem will be unlinked from smap if cmpxchg owner_storage_ptr
  fails, which should not fail. Therefore, hold b->lock when linking
  until allocation complete. Helpers that assume b->lock is held by
  callers are introduced: bpf_selem_link_map_nolock() and
  bpf_selem_unlink_map_nolock().

- bpf_local_storage_update()

  The three step update process: link_map(new_selem),
  link_storage(new_selem), and unlink_map(old_selem) should not fail in
  the middle.

In bpf_selem_unlink(), bpf_selem_unlink_map() and
bpf_selem_unlink_storage() should either all succeed or fail as a whole
instead of failing in the middle. So, return if unlink_map() failed.
Remove the selem_linked_to_map_lockless() check as an selem in the
common paths (not bpf_local_storage_map_free() or
bpf_local_storage_destroy()), will be unlinked under b->lock and
local_storage->lock and therefore no other threads can unlink the selem
from map at the same time.

In bpf_local_storage_destroy(), ignore the return of
bpf_selem_unlink_map() for now. A later patch will allow
bpf_local_storage_destroy() to unlink selems even when failing to
acquire locks.

Note that while this patch removes all callers of selem_linked_to_map(),
a later patch that introduces bpf_selem_unlink_nofail() will use it
again.

Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20260205222916.1788211-3-ameryhung@gmail.com

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