btrfs: allow fiemap to be interruptible

Doing fiemap on a file with a very large number of extents can take a very
long time, and we have reports of it being too slow (two recent examples
in the Link tags below), so make it interruptible.

Link: https://lore.kernel.org/linux-btrfs/21dd32c6-f1f9-f44a-466a-e18fdc6788a7@virtuozzo.com/
Link: https://lore.kernel.org/linux-btrfs/Ysace25wh5BbLd5f@atmark-techno.com/
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -5623,6 +5623,11 @@ int extent_fiemap(struct btrfs_inode *inode, struct fiemap_extent_info *fieinfo,
 				ret = 0;
 			goto out_free;
 		}
+
+		if (fatal_signal_pending(current)) {
+			ret = -EINTR;
+			goto out_free;
+		}
 	}
 out_free:
 	if (!ret)