Clear the data buffer length field when freeing a command structure so that
it doesn't leak through when the command structure is reused for a user
command without a data buffer.

PR:		amd64/189668
Tested by:	Pete Long <pete@nrth.org>
MFC after:	1 week
Bump .Dd, missed in r266206

Approved by:	hrs (mentor, implicit)
Correct documentation of the limit on how much memory can be mlock()ed

vm.max_wired is a system-wide limit, not per-process.  Reword the
section to make this more clear.

PR:		docs/189214
Submitted by:	Lawrence Chen (original text)
Approved by:	hrs (mentor)