fusefs: fallback to MKNOD/OPEN if a filesystem doesn't support CREATE

If a FUSE filesystem returns ENOSYS for FUSE_CREATE, then fallback to
FUSE_MKNOD/FUSE_OPEN.

Also, fix a memory leak in the error path of fuse_vnop_create. And do a
little cleanup in fuse_vnop_open.

PR: 199934
Reported by: samm@os2.kiev.ua
Sponsored by: The FreeBSD Foundation