selftests/bpf: Add tests for bpf_get_dentry_xattr

selftests/bpf: Add tests for bpf_get_dentry_xattr

Add test for bpf_get_dentry_xattr on hook security_inode_getxattr.
Verify that the kfunc can read the xattr. Also test failing getxattr
from user space by returning non-zero from the LSM bpf program.

Acked-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20240806230904.71194-4-song@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>