Possible VFS KPI and KBI breakage on stable/7

[Kostik Belousov]

There are three sets of changes that would benefit stable/7.
Namely, there are

1. Improvements for the UFS unmount or rw->ro remount, that perform
   suspension during the operation.

   The changes depend on the the suspension mechanism path,
   that introduced the suspension owner, and added new VFS OP
   into the mount method table.

   This might also fix the hangs with gjournal or gjournal together
   with snapshots experienced by some users.

   Since the only real consumer of the suspension is UFS, I believe that MFC
   would have quite low impact, if any.

   Corresponding revision is 183073.

2. The openat(2) and similar syscalls. The new ZFS requires openat()
   functionality. We have to change struct nameidata to merge NDINIT_ATVP().

   All modules using namei() need to be recompiled.

3. The Marcus' work on vn_fullpath() support for synthetic filesystems
   introduces new VOP, vop_vptocnp.

   This would allow procstat(1) to work on devfs and pseudofs vnodes.
   As I understand, this would also improve Gnome experience on FreeBSD.

   All fs modules need to be recompiled.

There was one very magisterial voice that objected against KBI breakage
on stable branch in principle. In my opinion, the benefits of the bug
fixes and functionality improvements with the proposed merges are much
greater then inconvenience of the need to recompile out-of-tree fs
modules. Changes were discussed with re@ to some extent.

In case there is vocal objection against the merge, I would abstain
from doing this.
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