generic/508: fix to check inode creation time feature on scratch mountpoint

generic/508: fix to check inode creation time feature on scratch mountpoint

_require_btime() just check inode creation time feature on TEST_DIR
mountpoint, but generic/508 needs to do that check on SCRATCH_MNT
mountpoint. Let's add _require_scratch_btime() for that, meanwhile
handling scratch_{mkfs,mount,umount} inside the function to decouple
with caller.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>