fusefs: set ctime during FUSE_SETATTR following a write

As of r349396 the kernel will internally update the mtime and ctime of files
on write. It will also flush the mtime should a SETATTR happen before the
data cache gets flushed. Now it will flush the ctime too, if the server is
using protocol 7.23 or higher.

This is the only case in which the kernel will explicitly set a file's
ctime, since neither utimensat(2) nor any other user interfaces allow it.

Sponsored by: The FreeBSD Foundation