wrappers for ->i_mutex access

parallel to mutex_{lock,unlock,trylock,is_locked,lock_nested},
inode_foo(inode) being mutex_foo(&inode->i_mutex).

Please, use those for access to ->i_mutex; over the coming cycle
->i_mutex will become rwsem, with ->lookup() done with it held
only shared.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/btrfs/xattr.c b/fs/btrfs/xattr.c
index fd953c3..6c68d63 100644
--- a/fs/btrfs/xattr.c
+++ b/fs/btrfs/xattr.c
@@ -126,7 +126,7 @@
 	 * locks the inode's i_mutex before calling setxattr or removexattr.
 	 */
 	if (flags & XATTR_REPLACE) {
-		ASSERT(mutex_is_locked(&inode->i_mutex));
+		ASSERT(inode_is_locked(inode));
 		di = btrfs_lookup_xattr(NULL, root, path, btrfs_ino(inode),
 					name, name_len, 0);
 		if (!di)