CHROMIUM: fs: bio: check bs null in error path

smatch error:
fs/bio.c:365 bio_alloc_bioset() error: we previously assumed 'bs' could be null (see line 329)
Added a check for if bs is null before freeing resources that may have been
assigned to bs.

BUG=chromium:237705
TEST=FEATURES=test emerge-$B chromeos-kernel-next

Change-Id: I1f1e30a1d095d5341584b5fec34e527c727c3f69
Signed-off-by: Paul Taysom <taysom@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/59069
Reviewed-by: Sonny Rao <sonnyrao@chromium.org>

fs/bio.c

@@ -490,7 +490,8 @@ struct bio *bio_alloc_bioset(gfp_t gfp_mask, int nr_iovecs, struct bio_set *bs)
 	return bio;
 
 err_free:
-	mempool_free(p, bs->bio_pool);
+	if (bs)
+		mempool_free(p, bs->bio_pool);
 	return NULL;
 }
 EXPORT_SYMBOL(bio_alloc_bioset);