zonefs: Always invalidate last cached page on append write

When a direct append write is executed, the append offset may correspond
to the last page of a sequential file inode which might have been cached
already by buffered reads, page faults with mmap-read or non-direct
readahead. To ensure that the on-disk and cached data is consistant for
such last cached page, make sure to always invalidate it in
zonefs_file_dio_append(). If the invalidation fails, return -EBUSY to
userspace to differentiate from IO errors.

This invalidation will always be a no-op when the FS block size (device
zone write granularity) is equal to the page size (e.g. 4K).

Reported-by: Hans Holmberg <Hans.Holmberg@wdc.com>
Fixes: 02ef12a6 ("zonefs: use REQ_OP_ZONE_APPEND for sync DIO")
Cc: stable@vger.kernel.org
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Tested-by: Hans Holmberg <hans.holmberg@wdc.com>