fstests: btrfs: Check if fallocate re-truncates page beyond EOF

fstests: btrfs: Check if fallocate re-truncates page beyond EOF

Even the fallocate range doesn't cover the last page of a file, btrfs
will still re-truncate the last page.

Such behavior is completely duplicated and unneeded, and fixed by the
following kernel patch:

btrfs: Avoid truncate tailing page if fallocate range doesn't exceed
inode size

Add this test case to check that malfunction.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>