aio: Fix a test and set race in aio_biowakeup.

Use atomic_fetchadd in place of separate atomic_subtract / atomic_load.

Reviewed by: markj
Sponsored by: HPE TidalScale
Differential Revision: https://reviews.freebsd.org/D38559

(cherry picked from commit 40734fc57ec0158690e9f03284229553b050b8ea)