xprtrdma: Reduce per-transport MR allocation

xprtrdma: Reduce per-transport MR allocation

[ Upstream commit 40c6ed0c8a7f2c5d67f5d6774bbce230a42176db ]

Reduce resource consumption per-transport to make way for increasing
the credit limit and maximum r/wsize. Pre-allocate fewer MRs.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Reviewed-by: Devesh Sharma <devesh.sharma@avagotech.com>
Tested-By: Devesh Sharma <devesh.sharma@avagotech.com>
Reviewed-by: Doug Ledford <dledford@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>