Btrfs: release path before starting transaction in can_nocow_extent

We can't be holding tree locks while we try to start a transaction, we will
deadlock.  Thanks,

Reported-by: Sage Weil <sage@inktank.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/inode.c

@@ -6437,6 +6437,7 @@ noinline int can_nocow_extent(struct inode *inode, u64 offset, u64 *len,
 
 	if (btrfs_extent_readonly(root, disk_bytenr))
 		goto out;
+	btrfs_release_path(path);
 
 	/*
 	 * look for other files referencing this extent, if we