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: NWei Yongjun <weiyj.lk@gmail.com>
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>