RDMA/uverbs: Refactor flags checks and update return value

Since commit f21519b2 ("IB/core: extended command: an
improved infrastructure for uverbs commands"), the uverbs
supports extra flags as an input to the command interface.

However actually, there is only one flag available and used,
so it is better to refactor the code, so the resolution and
report to the users is done as early as possible.

As part of this change, we changed the return value of failure case
from ENOSYS to be EINVAL to be consistent with the rest flags checks.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>