udf: Fix extending file within last block

When extending file within last block it can happen that the extent is
already rounded to the blocksize and thus contains the offset we want to
grow up to. In such case we would mistakenly expand the last extent and
make it one block longer than it should be, exposing unallocated block
in a file and causing data corruption. Fix the problem by properly
detecting this case and bailing out.

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