xfstests: add a new test case for ext4 indirect-based file

xfstests: add a new test case for ext4 indirect-based file

After applied this commit (864688d3), xfstests #255 will not test a
file system that cannot support fallocate(2), such as a indirect-based
file in ext4.  So we need to add a new generic test case to test it.

The difference between #255 and this test case is only to use pwrite to
allocate blocks.  Other filesystems should survive in this test case.
In the mean time, a new argument '-u' is added into _test_generic_punch
not to run unwritten tests.

Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
Reviewed-by: Rich Johnston <rjohnston@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>