For a fallocate's zero range operation that targets a range with an end
that is not aligned to the sector size, we can end up not updating the
inode's i_size. This happens when the last page of the range maps to an
unwritten (prealloc) extent and before that last page we have either a
hole or a written extent. This is because in this scenario we relied
on a call to btrfs_prealloc_file_range() to update the inode's i_size,
however it can only update the i_size to the "down aligned" end of the
range.

Example:

 $ mkfs.btrfs -f /dev/sdc
 $ mount /dev/sdc /mnt
 $ xfs_io -f -c "pwrite -S 0xff 0 428K" /mnt/foobar
 $ xfs_io -c "falloc -k 428K 4K" /mnt/foobar
 $ xfs_io -c "fzero 0 430K" /mnt/foobar
 $ du --bytes /mnt/foobar
 438272	/mnt/foobar

The inode's i_size was left as 428Kb (438272 bytes) when it should have
been updated to 430Kb (440320 bytes).
Fix this by always updating the inode's i_size explicitly after zeroing
the range.

Fixes: ba6d5887946ff86d93dc ("Btrfs: add support for fallocate's zero range operation")
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>