generic: test post-EOF gap zeroing persistence

generic: test post-EOF gap zeroing persistence

Test that extending a file past a non-block-aligned EOF correctly
zero-fills the gap [old_EOF, block_boundary), and that this zeroing
persists through a filesystem shutdown+remount cycle.

Stale data beyond EOF can persist on disk when append write data blocks
are flushed before the on-disk file size update, or when concurrent
append writeback and mmap writes persist non-zero data past EOF.
Subsequent post-EOF operations (append write, fallocate, truncate up)
must zero-fill and persist the gap to prevent exposing stale data.

The test pollutes the file's last physical block (via FIEMAP + raw
device write) with a sentinel pattern beyond i_size, then performs each
extend operation and verifies the gap is zeroed both in memory and on
disk.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Zorro Lang <zlang@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>