RDMA/rxe: Add unlocked versions of pool APIs

The existing pool APIs use the rw_lock pool_lock to protect critical
sections that change the pool state. This does not correctly implement a
typical sequence like the following

        elem = <lookup key in pool>

        if found use elem else

        elem = <alloc new elem in pool>

        <add key to elem>

Which is racy if multiple threads are attempting to perform this at the
same time. We want the second thread to use the elem created by the first
thread not create two equivalent elems.

This patch adds new APIs that are the same as existing APIs but do not
take the pool_lock. A caller can then take the lock and perform a sequence
of pool operations and then release the lock.

Link: https://lore.kernel.org/r/20201216231550.27224-7-rpearson@hpe.com
Signed-off-by: Bob Pearson <rpearson@hpe.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>