RDMA/rdmavt: Use int type to store negative error codes

RDMA/rdmavt: Use int type to store negative error codes

Change 'ret' from u32 to int in alloc_qpn() to store -EINVAL, and remove
the 'bail' label as it simply returns 'ret'.

Storing negative error codes in an u32 causes no runtime issues, but it's
ugly as pants,  Change 'ret' from u32 to int type - this change has no
runtime impact.

Signed-off-by: Qianfeng Rong <rongqianfeng@vivo.com>
Link: https://patch.msgid.link/20250826150556.541440-1-rongqianfeng@vivo.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>