RDMA/mana_ib: initialize err for empty send WR lists

RDMA/mana_ib: initialize err for empty send WR lists

mana_ib_post_send() returns err after walking the send work request list.
If the caller passes an empty list, the loop is skipped and err is not
assigned.

Initialize err to 0 so an empty send work request list returns success
instead of stack data.

Fixes: c8017f5b4856 ("RDMA/mana_ib: UD/GSI work requests")
Link: https://patch.msgid.link/r/20260618041752.481193-2-ruoyuw560@gmail.com
Signed-off-by: Ruoyu Wang <ruoyuw560@gmail.com>
Reviewed-by: Long Li <longli@microsoft.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>