smbd: split out a fsp_flush_write_time_update() function from update_write_time_handler()

smbd: split out a fsp_flush_write_time_update() function from update_write_time_handler()

It's confusing to call update_write_time_handler() from anywhere,
it should only be called from within the event loop when the
timer expires.

This makes it more obvious that fsp_flush_write_time_update()
doesn't really need an tevent context argument.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>