udf: Discard preallocation before extending file with a hole

When extending file with a hole, we tried to preserve existing
preallocation for the file. However that is not very useful and
complicates code because the previous extent may need to be rounded to
block boundary as well (which we forgot to do thus causing data
corruption for sequence like:

xfs_io -f -c "pwrite 0x75e63 11008" -c "truncate 0x7b24b" \
  -c "truncate 0xabaa3" -c "pwrite 0xac70b 22954" \
  -c "pwrite 0x93a43 11358" -c "pwrite 0xb8e65 52211" file

with 512-byte block size. Just discard preallocation before extending
file to simplify things and also fix this data corruption.

CC: stable@vger.kernel.org
Signed-off-by: Jan Kara <jack@suse.cz>