selftests/bpf: Extend test fs_kfuncs to cover security.bpf. xattr names

selftests/bpf: Extend test fs_kfuncs to cover security.bpf. xattr names

Extend test_progs fs_kfuncs to cover different xattr names. Specifically:
xattr name "user.kfuncs" and "security.bpf.xxx" can be read from BPF
program with kfuncs bpf_get_[file|dentry]_xattr(); while "security.bpf"
and "security.selinux" cannot be read.

Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20250130213549.3353349-3-song@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>