mesh: Fix FPE in overcommit logic.

mesh: Fix FPE in overcommit logic.

During overcommit, mesh_config_save is called in asynchronous mode to
avoid blocking Send() calls. This means that update of cfg->write_time
is scheduled via l_idle_oneshot, so if the next Send() gets scheduled
first, the code may see elapsed time of zero.

If this happens, then the overcommit logic was already executed and the
overcommit is pending, so we can just return.

Change-Id: I13bd46cd94e4a415ebcf2a97cff283bab2d635f4
Signed-off-by: anuj.bhumiya <anuj.bhumiya@samsung.com>