xprtrdma: Clean up synopsis of rpcrdma_req_create()

Commit 1769e6a8 ("xprtrdma: Clean up rpcrdma_create_req()")
added rpcrdma_req_create() with a GFP flags argument in case a
caller might want to avoid waiting for memory.

There has never been a caller that does not pass GFP_KERNEL as
the third argument. That argument can therefore be eliminated.

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