IB/srp: Dont allocate a page vector when using fast_reg

The new fast registration API does not reuqire a page vector
so we can't avoid allocating it.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Tested-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>