udf: don't increment lenExtents while writing to a hole

commit fb719c59bdb4fca86ee1fd1f42ab3735ca12b6b2 upstream.

Incrementing lenExtents even while writing to a hole is bad
for performance as calls to udf_discard_prealloc and
udf_truncate_tail_extent would not return from start if
isize != lenExtents

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Ashish Sangwan <a.sangwan@samsung.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>