Update comment regarding the locking of the sysctl tree.
Update comment regarding the locking of the sysctl tree.

Rename memlock to sysctllock, and MEMLOCK()/MEMUNLOCK() to SYSCTL_LOCK()/
SYSCTL_UNLOCK() and related changes to make the lock names make more
sense.

Submitted by:	Jonathan Mini <mini@haikugeek.com>

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