selftests/bpf: Add tests for filesystem kfuncs

Add selftests for two new filesystem kfuncs:
1. bpf_get_file_xattr
2. bpf_get_fsverity_digest

These tests simply make sure the two kfuncs work. Another selftest will be
added to demonstrate how to use these kfuncs to verify file signature.

CONFIG_FS_VERITY is added to selftests config. However, this is not
sufficient to guarantee bpf_get_fsverity_digest works. This is because
fsverity need to be enabled at file system level (for example, with tune2fs
on ext4). If local file system doesn't have this feature enabled, just skip
the test.

Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20231129234417.856536-6-song@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>