Btrfs: return error if the range we want to map is bogus

Currently if we have corrupt metadata map_extent_buffer will complain about it,
but not return an error so the caller has no idea a problem was hit.  Fix this.
Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>

fs/btrfs/extent_io.c

@@ -3690,6 +3690,7 @@ int map_private_extent_buffer(struct extent_buffer *eb, unsigned long start,
 		       "wanted %lu %lu\n", (unsigned long long)eb->start,
 		       eb->len, start, min_len);
 		WARN_ON(1);
+		return -EINVAL;
 	}
 
 	p = extent_buffer_page(eb, i);