IB/rxe: Refactor lookup memory function

Consolidate all error checks under single if() condition and use helper
unlikely() macro for them, in addition drop unneeded goto labels.

rxe_pool_get_index() already provides RB tree based efficient lookup.
Avoid doing extra checks for error cases which are rare and already
covered by rxe_pool_get_index().

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>