Add sysctl KERN_LOCKF

reporting the shapshot of the active advisory locks.

A new VFS ops method vfs_report_lockf if provided in the mount point
op table. If it is NULL, as it is currently for all existing
filesystems, vfs_report_lockf() function is used, which gathers
information from the standard implementation inside kern/kern_lockf.c.

Filesystems implementing its own locking (NFSv4 as example) can provide
a custom implementation.

Reviewed by: markj, rmacklem
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D34756