btrfs-progs: fix mkfs.btrfs -r option

Commit 605e806166 broke the
mkfs.btrfs -r option, because it calls make_btrfs
without ever setting dev_block_count, in the -r case,
so we tell it to make a filesystem of size 0.

Then we wander into ENOSPC land and segfault.

As a quick one-line-fix, just set the dev_block_count
to the size of the destination image file.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>

mkfs.c

@@ -1384,6 +1384,8 @@ int main(int ac, char **av)
 			fprintf(stderr, "unable to zero the output file\n");
 			exit(1);
 		}
+		/* our "device" is the new image file */
+		dev_block_count = block_count;
 	}
 
 	ssd = is_ssd(file);