vol: open() needs mode if called with O_CREAT

vol: open() needs mode if called with O_CREAT

If open() is called with a the O_CREAT flag, then it requires a
3rd argument, containing the mode with which to create the file.
On DARWIN (when we always use O_CREAT), and on other platforms
where the first call to open returned E_NOENT, we weren't doing this,
and so were presumably getting a random mode of whatever garbage was
on the stack.

Caught by clang-analyzer

Change-Id: Ic173e582e0d46f6d4cf8801dc7a6b4b393db5063
Reviewed-on: http://gerrit.openafs.org/2139
Reviewed-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>