dma-buf/udmabuf: Fix NULL pointer dereference in udmabuf_create

There is a potential execution path in which pointer memfd is NULL when
passed as argument to fput(), hence there is a NULL pointer dereference
in fput().

Fix this by null checking *memfd* before calling fput().

Addresses-Coverity-ID: 1473174 ("Explicit null dereferenced")
Fixes: fbb0de7950 ("Add udmabuf misc device")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20180904190749.GA9308@embeddedor.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

drivers/dma-buf/udmabuf.c

@@ -194,6 +194,7 @@ err_put_pages:
 	while (pgbuf > 0)
 		put_page(ubuf->pages[--pgbuf]);
 err_free_ubuf:
+	if (memfd)
 		fput(memfd);
 	kfree(ubuf->pages);
 	kfree(ubuf);