io_uring: rework io_uring_enter to simplify return value

io_uring_enter returns the count submitted preferrably over an error
code. In some code paths this check is not required, so reorganise the
code so that the check is only done as needed.
This is also a prep for returning error codes only in waiting scenarios.

Signed-off-by: Dylan Yudaken <dylany@fb.com>
Link: https://lore.kernel.org/r/20220421091345.2115755-4-dylany@fb.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>