generic/608: Add new regression test

generic/608: Add new regression test

Write data into a file and then enable DAX on the file immediately,
the written data which is still in the buffer should be synchronized
to disk instead of discarded when the corresponding inode is evicted.

It's a regression test for commit 88149082bb8e ("fs: Handle
I_DONTCACHE in iput_final() instead of generic_drop_inode()")

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