cxgbei: Explicitly clear the page pode reservation pointer after freeing it.

A single union ctl_io can be reused across multiple transfers (in
particular by the ramdisk backend). On a reuse, the reservation
pointer would retain its value from the previous transfer tripping an
assertion.

Reported by: Jithesh Arakkan @ Chelsio
Sponsored by: Chelsio Communications

Differential Revision: https://reviews.freebsd.org/D29939

(cherry picked from commit e894e3adb206815c2acff17a4011becb166c2f66)