generic: add mmap write vs truncate/remap test

generic: add mmap write vs truncate/remap test

This test exposed a problem with mapped writes to the tail page of a
file in XFS and potentially ext4. Eric did all the hard work of
taking the bug report and generating the reproducable test case on
ext4.  Make it a generic test so that we can ensure that all
filesystems handle the case correctly.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>