xprtrdma: Refactor FRMR invalidation

Clean up: After some recent updates, clarifications can be made to
the FRMR invalidation logic.

- Both the remote and local invalidation case mark the frmr INVALID,
  so make that a common path.

- Manage the WR list more "tastefully" by replacing the conditional
  that discriminates between the list head and ->next pointers.

- Use mw->mw_handle in all cases, since that has the same value as
  f->fr_mr->rkey, and is already in cache.

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