Merge branch 'add-bpf_get_dentry_xattr'

Song Liu says:

====================
Add bpf_get_dentry_xattr

Add a kfunc to read xattr from dentry. Also add selftest for the new
kfunc.

Changes v3 => v4:
1. Fix selftest build.

V3: https://lore.kernel.org/bpf/20240806203340.3503805-1-song@kernel.org/T/#u

Changes v2 => v3:
1. Move the kfuncs to fs/bpf_fs_kfuncs.c.
2. Fix selftests build error on s390. (Alexei)

v2: https://lore.kernel.org/bpf/20240730230805.42205-1-song@kernel.org/T/#u

Changes v1 => v2:
1. Remove 3 kfuncs that are ready yet.

v1: https://lore.kernel.org/linux-fsdevel/20240725234706.655613-1-song@kernel.org/T/#u
====================

Link: https://lore.kernel.org/r/20240806230904.71194-1-song@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>