xprtrdma: Rename rpcrdma_frwr::sg and sg_nents

Clean up: Follow same naming convention as other fields in struct
rpcrdma_frwr.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Tested-by: Steve Wise <swise@opengridcomputing.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>