common/fuzzy: make _scratch_fuzz_modify work for non-xfs filesystems

common/fuzzy: make _scratch_fuzz_modify work for non-xfs filesystems

Since commit 9bab148bb3c7 ("common/fuzzy: exercise the filesystem a little
harder after repairing") funtion _scratch_fuzz_modify() has become
xfs-specific due to the use of some functions that assume this filesytem,
namely _xfs_force_bdev() and _xfs_has_feature().

Ensure _scratch_fuzz_modify() works again with other filesystems by using
these functions only when testing xfs.

Signed-off-by: "Luis Henriques (SUSE)" <luis.henriques@linux.dev>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>