generic: Verify if statx() can qurey S_DAX flag on regular file correctly

generic: Verify if statx() can qurey S_DAX flag on regular file correctly

1) With new kernel(e.g. v5.8-rc1), statx() can be used to qurey S_DAX flag
   on regular file, so add a test to verify the feature.
2) Factor out _check_s_dax() so that other tests can use it in future.

Reference:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=712b2698e4c024b561694cbcc1abba13eb0fd9ce

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