xprtrdma: Fix a maybe-uninitialized compiler warning

This minor fix-up keeps GCC from complaining that "last' may be used
uninitialized", which breaks some build workflows that have been running
with all warnings treated as errors.

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>