xprtrdma: Fix I3 invariant comment in rpcrdma_complete_rqst

xprtrdma: Fix I3 invariant comment in rpcrdma_complete_rqst

frwr_unmap_sync() and frwr_unmap_async() drain rl_registered via
rpcrdma_mr_pop() before posting invalidation Work Requests to
hardware.  The WARN_ON_ONCE verifies that the list-drain step
has occurred, not that hardware unmapping has completed.

Reword the comment to match what the assertion actually checks.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <anna.schumaker@hammerspace.com>