selftests/xattr: path-based AF_UNIX socket xattr tests

selftests/xattr: path-based AF_UNIX socket xattr tests

Test user.* extended attribute operations on path-based Unix domain
sockets (SOCK_STREAM, SOCK_DGRAM, SOCK_SEQPACKET). Path-based sockets
are bound to a filesystem path and their inodes live on the underlying
filesystem (e.g. tmpfs).

Covers set/get/list/remove, persistence, XATTR_CREATE/XATTR_REPLACE
flags, empty values, size queries, buffer-too-small errors, O_PATH fd
operations, and trusted.* xattr handling.

Link: https://patch.msgid.link/20260216-work-xattr-socket-v1-12-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>