MFC r350067:

Add missing mode in open(2) calls with O_CREAT.

When O_CREAT is specified, the third, variadic argument is
required as the permission. If on is not passed, then depending
on the ABI, either the contents of the third argument register
or some arbitrary stuff on the stack will be used as the permission.

This has been merged to NetBSD.

Reviewed by: asomers, ngie
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D20972