RDMA/rxe: Fix bug in rxe_alloc()

A recent patch which added an 'unlocked' version of rxe_alloc introduced a
bug causing kzalloc(..., GFP_KERNEL) to be called while holding a spin
lock. This patch corrects that error.

rxe_alloc_nl() should always be called while holding the pool->pool_lock
so the 2nd argument to kzalloc there should be GFP_ATOMIC.

rxe_alloc() prior to the change only locked the code around checking that
pool->state is RXE_POOL_STATE_VALID to avoid races between working threads
and a thread shutting down the rxe driver. This patch reverts rxe_alloc()
to this behavior so the lock is not held when kzalloc() is called.

Link: https://lore.kernel.org/r/20210125211641.2694-2-rpearson@hpe.com
Reported-by:  <syzbot+ec2fd72374785d0e558e@syzkaller.appspotmail.com>
Fixes: 3853c35e

 ("RDMA/rxe: Add unlocked versions of pool APIs")
Signed-off-by: Bob Pearson <rpearson@hpe.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>