vfs: fixup WANTIOCTLCAPS on open

In some cases vn_open_cred overwrites cn_flags, effectively nullifying
initialisation done in NDINIT. This will have to be fixed.

In the meantime make sure the flag is passed.

Reported by: jenkins
Noted by: Mathieu <sigsys@gmail.com>
Approved by: re (gjb)

(cherry picked from commit b7262756e2f471f6481070e2473af7853506b150)
(cherry picked from commit 792ebbb1557e290805d84b87653110681c7338a0)