ffs: fix mksnap_ffs

[rew]

Summary

Take a snapshot of the filesystem as-is and return without altering
ro/rw, or any other options that may be passed in from mount.

This changes the behavior of mount. A specific example, is when trying
to upgrade from ro->rw and taking a snapshot at the same time.

mount -u -w -o snapshot /snapshot /

If breaking the above behavior is unacceptable, another approach I thought of
was to put the relevant namei/vnode bits behind if statements, something like:

if (!MNT_SNAPSHOT)
    /* do namei() stuff */

It seems a bit hacky, but wouldn't change current behavior, so that
approach might be preferable.

A third approach could be to revert the commit that introduced this
breakage, and solve the original bug in a different way.

Or...am I missing an obvious solution?

[pho]

I have been running tests with this patch for 18 hours, without seeing any problems.

[mckusick]

The main clients of making snapshots are dump(8), fsck_ffs(8), and mksnap_ffs(8) and none of them want to change any other attributes. Generally speaking, administrators do not use the mount(8) command directly to take snapshots. The one other change I would recommend is to document in the mount.8 manual page that when the "snapshot" option is specified, any other options will be ignored.