generic: mmap and copy file data with page overlapping

generic: mmap and copy file data with page overlapping

Mmap 2 pages of file, write 64 bytes to the first and second pages,
copy the data from the first page and then second page to the second
page offset with $pagesize - 64. Verify the data at the end.

       +-----------------------+
       |        (copy)         |
       |                       V
    +---------------+---------------+------------
    |AAAA| ........ |AAAA| ... |AAAA|AAAA|
    +---------------+---------------+------------
                       |            ^
                       |   (copy)   |
                       +------------+

This's also a regression test cover kernel commit: 4f06dd92b5d0
("fuse: fix write deadlock")

Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>