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Mostly revert a5970a529c2d95271: Make files opened with O_PATH to not block non…

Description

Mostly revert a5970a529c2d95271: Make files opened with O_PATH to not block non-forced unmount

Problem is that open(O_PATH) on nullfs -o nocache is broken then,
because there is no reference on the vnode after the open syscall exits.

Reported and tested by: ambrisko
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week