udf: Allocate blocks on write page fault

Currently if file with holes is mapped, udf allocates blocks for dirtied
pages during page writeback. This however creates problems when to
truncate final extent to proper size and currently we leave the last
extent untruncated which violates UDF standard. So allocate blocks on
write page fault instead. In that case the last extent gets truncated
the file is closed and everything is happy.

Signed-off-by: Jan Kara <jack@suse.cz>