libceph: fix return value check in alloc_msg_with_page_vector()

In case of error, the function ceph_alloc_page_vector() returns
ERR_PTR() and never returns NULL. The NULL test in the return value
check should be replaced with IS_ERR().

Fixes: 19079203 ('libceph: support for sending notifies')
Signed-off-by: Wei Yongjun <weiyj.lk@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>