Add a sleep count for freebsd_umtx_sem2_wait/wake().

This change adds a sleep count for freebsd_umtx_sem2_wait() and
freebsd_umtx_sem2_wake() by using the unused upper 30 bits of the
_flag field of the semaphore struct. The lock count is used to
know when to clear the USEM_HAS_WAITERS bit in the count field.