xattr handlers: plug a lock leak in simple_xattr_list

The code could leak xattrs->lock on error.

Problem introduced with 786534b9

 "tmpfs: listxattr should
include POSIX ACL xattrs".

Signed-off-by: Mateusz Guzik <mguzik@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>