RDMA: Use rlimit helpers

Make sure compiler won't do weird things with limits by using the
rlimit helpers added in 3e10e716 ("resource: add helpers for fetching
rlimits").  E.g. fetching them twice may return 2 different values
after writable limits are implemented.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Roland Dreier <rolandd@cisco.com>