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/inode.c b/fs/btrfs/inode.c
index 1b79dc9..e28f3d4 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -8447,7 +8447,7 @@
 		 * not unlock the i_mutex at this case.
 		 */
 		if (offset + count <= inode->i_size) {
-			mutex_unlock(&inode->i_mutex);
+			inode_unlock(inode);
 			relock = true;
 		}
 		ret = btrfs_delalloc_reserve_space(inode, offset, count);
@@ -8504,7 +8504,7 @@
 	if (wakeup)
 		inode_dio_end(inode);
 	if (relock)
-		mutex_lock(&inode->i_mutex);
+		inode_lock(inode);
 
 	return ret;
 }