kern/60026: zfs mount issues

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>Number:         60026
>Category:       kern
>Synopsis:       zfs mount issues
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Feb 22 04:15:00 +0000 2026
>Originator:     YAMAMOTO Takashi
>Release:        NetBSD 10.1
>Organization:
>Environment:
amd64
>Description:
* zmount() in libzfs always passes 0 to mount(2) 'flags' argument
* zmount() always sets 'uap->flags' to 0.
* zmount() stores something in uap->mflag. but no one seems to use it.
* zmount() sets up uap->optptr, but it doesn't set MS_OPTIONSTR.
* we don't implement MS_OPTIONSTR anyway.
* because zmount() effectively discards most of options,
  it's a bit adventurous to use -o in "zfs mount" and mount_zfs.

>How-To-Repeat:
eg.
zfs mount -o remount y/tmp
>Fix:
a. implement MS_OPTIONSTR in our vfs (not zfs, because some options would need to be cared before getting into zfs)

b. or implement it in libzfs (eg. translate them to our MNT_UPDATE etc.)

either ways, mount ABI changes, at least for zfs.