unmount: resume logging of stdout and stderr for filtering

unmount: resume logging of stdout and stderr for filtering

There's a number of places where a test program calls a variant of
_unmount but then pipes the output through a _filter script or
something.  The new _unmount helper redirects stdout and stderr to
seqres.full, which means that those error messages (some of which are
encoded in the golden outputs) are suppressed.  This leads to test
regressions in generic/050 and other places, so let's resume logging.

This also undoes all the changes that removed /dev/null redirection of
unmount calls.

Cc: <fstests@vger.kernel.org> # v2024.12.08
Fixes: 4c6bc4565105e6 ("fstests: clean up mount and unmount operations")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>