btrfs: allow hole and data seeking to be interruptible

Doing hole or data seeking on a file with a very large number of extents
can take a long time, and we have reports of it being too slow (such as
at LSFMM from 2017, see the Link below). So make it interruptible.

Link: https://lwn.net/Articles/718805/
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/file.c

@@ -3652,6 +3652,10 @@ static loff_t find_desired_extent(struct btrfs_inode *inode, loff_t offset,
 		start = em->start + em->len;
 		free_extent_map(em);
 		em = NULL;
+		if (fatal_signal_pending(current)) {
+			ret = -EINTR;
+			break;
+		}
 		cond_resched();
 	}
 	free_extent_map(em);