Fix compat10 semaphore interface race

Wrong has-waiters and missing unconditional _count==0 check may cause
infinite waiting with already non-zero count.

1. properly clear _has_waiters flag when waiting failed to start
2. always check _count before start waiting

PR: 265997
Reviewed by: kib
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D36272