Btrfs: avoid sleeping in verify_parent_transid while atomic

verify_parent_transid needs to lock the extent range to make
sure no IO is underway, and so it can safely clear the
uptodate bits if our checks fail.

But, a few callers are using it with spinlocks held.  Most
of the time, the generation numbers are going to match, and
we don't want to switch to a blocking lock just for the error
case.  This adds an atomic flag to verify_parent_transid,
and changes it to return EAGAIN if it needs to block to
properly verifiy things.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent-tree.c

@@ -6568,7 +6568,7 @@ static noinline int do_walk_down(struct btrfs_trans_handle *trans,
 			goto skip;
 	}
 
-	if (!btrfs_buffer_uptodate(next, generation)) {
+	if (!btrfs_buffer_uptodate(next, generation, 0)) {
 		btrfs_tree_unlock(next);
 		free_extent_buffer(next);
 		next = NULL;