udmabuf: fix general protection fault in udmabuf_create

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 f6974c7db627 ("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: f6974c7db627 ("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>