RDMA/rxe: Optimize the mr pool struct

Based on the commit c9f4c695 ("RDMA/rxe: Reverse the sense of
RXE_POOL_NO_ALLOC"), only the mr pool uses the RXE_POOL_ALLOC, As such,
replace this flags with pool type to save memory.

Link: https://lore.kernel.org/r/20220428041028.1363139-1-yanjun.zhu@linux.dev


Signed-off-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>