fusefs: sanitize FUSE_READLINK results for embedded NULs

If VOP_READLINK returns a path that contains a NUL, it will trigger an
assertion in vfs_lookup. Sanitize such paths in fusefs, rejecting any
and warning the user about the misbehaving server.

PR: 274268
MFC after: 1 week
Sponsored by: Axcient
Reviewed by: mjg, markj
Differential Revision: https://reviews.freebsd.org/D42081