xprtrdma: Clean up rpcrdma_create_req()

Eventually, I'd like to invoke rpcrdma_create_req() during the
call_reserve step. Memory allocation there probably needs to use
GFP_NOIO. Therefore a set of GFP flags needs to be passed in.

As an additional clean up, just return a pointer or NULL, because
the only error return code here is -ENOMEM.

Lastly, clean up the function names to be consistent with the
pattern: "rpcrdma" _ object-type _ action

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