fusefs: validate servers' error values

Formerly fusefs would pass up the stack any error value returned by the
fuse server. However, some values aren't valid for userland, but have
special meanings within the kernel. One of these, EJUSTRETURN, could
cause a kernel page fault if the server returned it in response to
FUSE_LOOKUP. Fix by validating all errors returned by the server.

Also, fix a data lifetime bug in the FUSE_DESTROY test.

PR: 263220
Reported by: Robert Morris <rtm@lcs.mit.edu>
MFC after: 3 weeks
Sponsored by: Axcient
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D34931