IB/rdmavt: Avoid reseting wqe send_flags in unreserve

The wqe should be read only and in fact the superfluous reset of the
RVT_SEND_RESERVE_USED flag causes an issue where reserved operations
elicit a bad completion to the ULP.

The maintenance of the flag is now entirely within rvt_post_one_wr()
where a reserved operation will set the flag and a non-reserved operation
will insure the operation that is about to be posted has the flag reset.

Fixes: Commit 856cc4c2 ("IB/hfi1: Add the capability for reserved operations")
Reviewed-by: Don Hiatt <don.hiatt@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>