generic: test attempt to reflink eof block into the middle of a file

generic: test attempt to reflink eof block into the middle of a file

Test that we can not clone a range from a file A into the middle of a file B
when the range includes the last block of file A and file A's size is not
aligned with the filesystem's block size. Allowing such case would lead to
data corruption since the data between EOF and the end of its block is
undefined.

This is motivated by a bug recently found that affects both Btrfs and XFS
and is fixed by the following commits/patches for the linux kernel:

 07d19dc9fbe9 ("vfs: avoid problematic remapping requests into partial EOF block")
 b39989009bdb ("xfs: fix data corruption w/ unaligned reflink ranges")
 Btrfs: fix data corruption due to cloning of eof block

The VFS patch landed in kernel 4.20-rc1 and the XFS patch landed in 4.19.
The Btrfs fix is very recent and it is not yet in Linus' tree.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>