vfs: Don't clobber namei flags in vn_open_cred()

Otherwise NAMEILOOKUP is cleared. More generally it seems quite
surprising that the flags set by vn_open_cred() callers are not
automatically preserved. Modify open2nameif() such that it takes
already-set namei flags into account.

Reviewed by: olce, kib
Fixes: 7587f6d4840f ("namei: Make stackable filesystems check harder for jail roots")
Differential Revision: https://reviews.freebsd.org/D50531