Pass a valid mode with O_CREATE to open(2).

CheriABI is pickier about the arguments to open(2) and crashes with a
fault if a mode isn't passed to an open() when O_CREATE is specified.

Reported by: CHERI
Reviewed by: brooks
Obtained from: CheriBSD
MFC after: 2 weeks
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D26281