RDMA/rxe,siw: Restore uverbs_cmd_mask IB_USER_VERBS_CMD_POST_SEND

These two drivers open code the call to POST_SEND and do not use the
rdma-core wrapper to do it, thus their usages was missed during the audit.

Both drivers use this as a doorbell to signal the kernel to start DMA.

Fixes: 628c02bf

 ("RDMA: Remove uverbs cmds from drivers that don't use them")
Link: https://lore.kernel.org/r/0-v1-4608c5610afa+fb-uverbs_cmd_post_send_fix_jgg@nvidia.com
Reported-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>