xprtrdma: Avoid Send Queue wrapping

Send WRs can be signalled or unsignalled. A signalled Send WR
always has a matching Send completion, while a unsignalled Send
has a completion only if the Send WR fails.

xprtrdma has a Send account mechanism that is designed to reduce
the number of signalled Send WRs. This in turn mitigates the
interrupt rate of the underlying device.

RDMA consumers can't leave all Sends unsignaled, however, because
providers rely on Send completions to maintain their Send Queue head
and tail pointers. xprtrdma counts the number of unsignaled Send WRs
that have been posted to ensure that Sends are signalled often
enough to prevent the Send Queue from wrapping.

This mechanism neglected to account for FastReg WRs, which are
posted on the Send Queue but never signalled. As a result, the
Send Queue wrapped on occasion, resulting in duplication completions
of FastReg and LocalInv WRs.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>