[Bug 254058] zfs doesn't set mnt_time aka RootOnZFS doesn't work without RTC

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=254058

            Bug ID: 254058
           Summary: zfs doesn't set mnt_time aka RootOnZFS doesn't work
                    without RTC
           Product: Base System
           Version: Unspecified
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: peterj at FreeBSD.org

vfs_mountroot() extracts the most recent mnt_time from the mounted filesystems
(root and devfs at the time) and passes it to inittodr() as a fallback if there
isn't a functioning system RTC.  This is intended to ensure that things like
log timestamps don't go backwards.

Unfortunately, only the UFS code actually initialises mnt_time so other root
filesystems types will leave mnt_time set to the epoch (1970-01-01 0000Z). 
Root on NFS isn't impacted because that uses nfs_mountroot(), which does
inittodr() based on the remote filesystem.

That leaves ZFS: If a system with root on ZFS doesn't have a functioning RTC
then the system time will start at 1970-01-01 0000Z until (eg) NTP fixes it. 
This leaves close-to-epoch timestamps strewn throughout the filesystem, which
is annoying.

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