common/rc: Update _exclude_scratch_mount_option() and _require_dm_target() for new...

common/rc: Update _exclude_scratch_mount_option() and _require_dm_target() for new dax option

Some tests(e.g. ext4/035) cannot include dax option(dax=inode/dax=never
is OK) so make _exclude_scratch_mount_option() and _require_dm_target()
check if old dax or new dax=always option is not defined in $MOUNT_OPTIONS.

Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>