udf: Move setting of i_lenExtents into udf_do_extend_file()

When expanding file for a write into a hole, we were not updating total
length of inode's extents properly. Move the update of i_lenExtents into
udf_do_extend_file() so that both expanding of file by truncate and
expanding of file by writing beyond EOF properly update the length of
extents. As a bonus, we also correctly update the length of extents when
only part of extents can be written.

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