generic: test zero page cache beyond new EOF on truncate down

generic: test zero page cache beyond new EOF on truncate down

From mmap(2) manpage, "a file is mapped in multiples of the page
size. For a file that is not a multiple of the page size, the
remaining memory is zeroed when mapped", this test is to test this
behavior on truncate down.

This is inspired by an XFS bug that truncate down fails to zero page
cache beyond new EOF and causes stale data written to disk
unexpectedly and a subsequent mmap sees non-zeros post EOF.

Patch "xfs: truncate pagecache before writeback in
xfs_setattr_size()" fixed the bug on XFS.

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>