udf: Init/maintain file entry checkpoint field

In accordance with ECMA 1.67 Part 4, 14.9.15, the checkpoint field
should be initialized to 1 at creation. (Zero is *not* a valid value.)

Signed-off-by: Steven P. Nickel <snickel@focusinfo.com>
Signed-off-by: Jan Kara <jack@suse.cz>