LU-9859 ptlrpc: change imp_refcount to refcount_t

The lustre portals handle was removed from the OBD import so
it missed the move to refcount_t with the LU-12452 work. Moving
to refcount_t also gives us security protection due to overflow
issues with atomic_t. Lastly refcount_t gives use the same
equivalent as LASSERT_ATOMIC_* with CONFIG_REFCOUNT_FULL thus
allowing us to remove the LASSERT_ATOMIC_* wrappers in time.

Change-Id: Ifdbd51c39bd3921e3f5d18d60efed6d1ff58c5c6
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/37733
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>