xprtrdma: Move rpcrdma_mr_put()

Clean up: This function is now invoked only in frwr_ops.c. The move
enables deduplication of the trace_xprtrdma_mr_unmap() call site.

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