RDMA/core: Add FRMR pools statistics

RDMA/core: Add FRMR pools statistics

Count for each pool the number of FRMR handles popped and held by user
MRs.
Also keep track of the max value of this counter.

Next patches will expose the statistics through netlink.

Signed-off-by: Michael Guralnik <michaelgur@nvidia.com>
Reviewed-by: Yishai Hadas <yishaih@nvidia.com>
Signed-off-by: Edward Srouji <edwards@nvidia.com>
Link: https://patch.msgid.link/20260226-frmr_pools-v4-4-95360b54f15e@nvidia.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>