When open(2) was invoked against a FUSE filesystem with an unexpected flags
value (no O_RDONLY / O_RDWR / O_WRONLY), an assertion fired, causing panic.
For now, prevent the panic by rejecting such VOP_OPENs with EINVAL.
This is not considered the correct long term fix, but does prevent an
unprivileged denial-of-service.
PR: 236329
Reported by: asomers
Reviewed by: asomers
Sponsored by: Dell EMC Isilon
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