xprtrdma: Fall back to MTHCAFMR when FRMR is not supported

An audit of in-kernel RDMA providers that do not support the FRMR
memory registration shows that several of them support MTHCAFMR.
Prefer MTHCAFMR when FRMR is not supported.

If MTHCAFMR is not supported, only then choose ALLPHYSICAL.

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