CIFS: check if pages is null rather than bv for a failed allocation

pages is being allocated however a null check on bv is being used
to see if the allocation failed. Fix this by checking if pages is
null.

Detected by CoverityScan, CID#1432974 ("Logically dead code")

Fixes: ccf7f4088af2dd ("CIFS: Add asynchronous context to support kernel AIO")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <smfrench@gmail.com>

fs/cifs/misc.c

@@ -810,7 +810,7 @@ setup_aio_ctx_iter(struct cifs_aio_ctx *ctx, struct iov_iter *iter, int rw)
 
 	if (!pages) {
 		pages = vmalloc(max_pages * sizeof(struct page *));
-		if (!bv) {
+		if (!pages) {
 			kvfree(bv);
 			return -ENOMEM;
 		}