xattr: fix kernel-doc for mnt_userns and vfs xattr helpers

xattr: fix kernel-doc for mnt_userns and vfs xattr helpers

Fix kernel-doc warnings in xattr.c:

../fs/xattr.c:257: warning: Function parameter or member 'mnt_userns' not described in '__vfs_setxattr_locked'
../fs/xattr.c:485: warning: Function parameter or member 'mnt_userns' not described in '__vfs_removexattr_locked'

and fix one function whose kernel-doc was not in the correct format.

Link: https://lore.kernel.org/r/20210216042929.8931-4-rdunlap@infradead.org
Fixes: 71bc356f93a1 ("commoncap: handle idmapped mounts")
Fixes: b1ab7e4b2a88 ("VFS: Factor out part of vfs_setxattr so it can be called from the SELinux hook for inode_setsecctx.")
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>
Cc: David P. Quigley <dpquigl@tycho.nsa.gov>
Cc: James Morris <jmorris@namei.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>