selftests/xattr: test xattrs on various socket families

selftests/xattr: test xattrs on various socket families

Test user.* xattr operations on sockets from different address families:
AF_INET, AF_INET6, AF_NETLINK, and AF_PACKET. All socket types use
sockfs for their inodes, so user.* xattrs should work regardless of
address family.

Each fixture creates a socket (no bind needed) and verifies the full
fsetxattr/fgetxattr/flistxattr/fremovexattr cycle. AF_INET6 skips if
not supported; AF_PACKET skips if CAP_NET_RAW is unavailable.

Also tests abstract namespace AF_UNIX sockets, which live in sockfs
(not on a filesystem) and should support user.* xattrs.

Link: https://patch.msgid.link/20260216-work-xattr-socket-v1-14-c2efa4f74cb7@kernel.org
Acked-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>