Fix logic error in the description of counter_u64_alloc: the command
may fail only if M_NOWAIT is specified, not M_WAITOK.
Reviewed by: glebius
MFC after: 2 weeks
Sponsored by: Spectra Logic Corporation
Introduce counter(9) API, that implements fast and raceless counters,
provided (but not limited to) for gathering of statistical data.
See http://lists.freebsd.org/pipermail/freebsd-arch/2013-April/014204.html
for more details.
In collaboration with: kib
Reviewed by: luigi
Tested by: ae, ray
Sponsored by: Nginx, Inc.
