IB/rxe: check for allocation failure on elem

The allocation for elem may fail (especially because we're using
GFP_ATOMIC) so best to check for a null return.  This fixes a potential
null pointer dereference when assigning elem->pool.

Detected by CoverityScan CID#1357507 ("Dereference null return value")

Fixes: 8700e3e7 ("Soft RoCE driver")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>