libfs: fix kernel-doc for mnt_userns

Fix kernel-doc warning in libfs.c.

../fs/libfs.c:498: warning: Function parameter or member 'mnt_userns' not described in 'simple_setattr'

Link: https://lore.kernel.org/r/20210216042929.8931-2-rdunlap@infradead.org/
Fixes: 549c729 ("fs: make helpers idmap mount aware")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

Author: rddunlap

fs/libfs.c

@@ -481,6 +481,7 @@ EXPORT_SYMBOL(simple_rename);
 
 /**
  * simple_setattr - setattr for simple filesystem
+ * @mnt_userns: user namespace of the target mount
  * @dentry: dentry
  * @iattr: iattr structure
  *