xprtrdma: Remove request_module from backchannel

Since commit ffe1f0df ("rpcrdma: Merge svcrdma and xprtrdma
modules into one"), the forward and backchannel components are part
of the same kernel module. A separate request_module() call in the
backchannel code is no longer necessary.

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