xprtrdma: Rename fields in rpcrdma_fmr

Clean up: Use the same naming convention used in other
RPC/RDMA-related data structures.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Tested-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>