selftests/xattr: sockfs socket xattr tests

Test user.* extended attribute operations on sockfs sockets. Sockets
created via socket() have their inodes in sockfs, which now supports
user.* xattrs with per-inode limits.

Tests fsetxattr/fgetxattr/flistxattr/fremovexattr operations including
set/get, listing (verifies system.sockprotoname presence), remove,
update, XATTR_CREATE/XATTR_REPLACE flags, empty values, size queries,
and buffer-too-small errors.

Also tests per-inode limit enforcement: maximum 128 xattrs, maximum
128KB total value size, limit recovery after removal, and independent
limits across different sockets.

Link: https://patch.msgid.link/20260216-work-xattr-socket-v1-13-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>

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