fusefs: forward UTIME_NOW to the server

If a user sets both atime and mtime to UTIME_NOW when calling a syscall like
utimensat(2), allow the server to choose what "now" means. Due to the
design of FreeBSD's VFS, it's not possible to do this for just one of atime
or mtime; it's all or none.

PR: 237181
Sponsored by: The FreeBSD Foundation