fstests: clean up mount and unmount operations

fstests: clean up mount and unmount operations

The way tests run unmount is, at times, completely random.
Sometimes they call the correct _scratch_unmount function, sometimes
they open code it with a direct call to UMOUNT_PROG <dir>, sometimes
they run umount directly.

This makes it really hard to instrument unmount operations when
trying to work out why transient, unpredictable failures like
this occur randomly during a test run:

umount: /mnt/xfs/runner-17/test: target is busy.

Sometimes it happens on a test device mount, sometimes a scratch
device mount.  Sometimes it happens to a test specific dm or loop
device mount. But without instrumenting every single unmount call in
every test, it's impossible to capture these failures easily.

Solve this problem by introducing the _unmount() wrapper. It is
simply a call to UMOUNT_PROG <dir>, but it provides a single point
were -every- unmount operation funnels through.

We already have a _mount wrapper for this reason. However, in trying
to work out why mounts were failing (because unmounts were failing),
I discovered that that_mount() is used inconsistently as well.

Sort this all out by adding and _unmount() wrapper to go with
_mount() and use them everywhere consistently.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Zorro lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>