fusefs: WIP supporting -o default_permissions

fusefs: WIP supporting -o default_permissions

Normally all permission checking is done in the fuse server.  But when -o
default_permissions is used, it should be done in the kernel instead.  This
commit adds appropriate permission checks through fusefs when -o
default_permissions is used.  However, sticky bit checks aren't working yet.
I'll handle those in a follow-up commit.

There are no checks for file flags, because those aren't supported by our
version of the FUSE protocol.  Nor is there any support for ACLs, though
that could be added if there were any demand.

PR:		216391
Reported by:	hiyorin@gmail.com
Sponsored by:	The FreeBSD Foundation