udmabuf: fix general protection fault in udmabuf_create

Syzbot reported general protection fault in udmabuf_create. The problem
was in wrong error handling.

In commit 16c243e ("udmabuf: Add support for mapping hugepages (v4)")
shmem_read_mapping_page() call was replaced with find_get_page_flags(),
but find_get_page_flags() returns NULL on failure instead PTR_ERR().

Wrong error checking was causing GPF in get_page(), since passed page
was equal to NULL. Fix it by changing if (IS_ER(!hpage)) to if (!hpage)

Reported-by: syzbot+e9cd3122a37c5d6c51e8@syzkaller.appspotmail.com
Fixes: 16c243e ("udmabuf: Add support for mapping hugepages (v4)")
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20210811175052.21254-1-paskripkin@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

drivers/dma-buf/udmabuf.c

@@ -227,8 +227,8 @@ static long udmabuf_create(struct miscdevice *device,
 				if (!hpage) {
 					hpage = find_get_page_flags(mapping, pgoff,
 								    FGP_ACCESSED);
-					if (IS_ERR(hpage)) {
-						ret = PTR_ERR(hpage);
+					if (!hpage) {
+						ret = -EINVAL;
 						goto err;
 					}
 				}