kernel: fix mailbox usage of SYS_DLIST_FOR_EACH_NODE()

SYS_DLIST_FOR_EACH_NODE() is marked as non-safe when an item is removed
from the list while looping over it. This is not true per-se, since the
item, when removed, keeps its next and prev pointers intact; however, it
is true if the item is then put into a list, be it a different one or
the same one. To prevent this, SYS_DLIST_FOR_EACH_NODE_SAFE() must be
used.

_mbox_message_put() can remove items from the rx queue and then put them
in the ready queue: this would cause the loop to start processing other
ready threads as item in the rx queue.

k_mbox_get() also removes items, from the tx queue, but does not seem to
add them to another list; however, it now uses the safe version as well,
since that is the proper usage.

Change-Id: Ieccbff238fc8a036c0d53d873eaaf55f4f5a14af
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>