generic: test that mmap-write updates c/mtime

generic: test that mmap-write updates c/mtime

When using mmap() for file i/o, writing to the file should update
it's c/mtime. Specifically if we first mmap-read from a page, then
memap-write to the same page.

This test was failing for the initial submission of DAX because
pfn based mapping do not have an page_mkwrite called for them.
The new Kernel patches that introduce pfn_mkwrite fixes this test.

Test adapted from a script originally written by Dave Chinner.

Signed-off-by: Omer Zilberberg <omzg@plexistor.com>
Signed-off-by: Boaz Harrosh <boaz@plexistor.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>