fuzzy: don't fail on compressed metadumps

This line in __scratch_xfs_fuzz_mdrestore:

	test -e "${POPULATE_METADUMP}"

Breaks spectacularly on a setup that uses DUMP_COMPRESSOR to compress
the metadump files, because the metadump files get the compression
program added to the name (e.g. "${POPULATE_METADUMP}.xz").  The check
is wrong, and since the naming policy is an implementation detail of
_xfs_mdrestore, let's get rid of the -e test.

However, we still need a way to fail the test if the metadump cannot be
restored.  _xfs_mdrestore returns nonzero on failure, so use that
instead.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>