Bluetooth: Mesh: Fix RPL fragmentation

`bt_mesh_rpl_check` stops iterating `replay_list` if either it found an
entry with the requested source address or unassigned address. When IV
index updated, `bt_mesh_rpl_reset` is called. It will set `old_iv` to 1
for all entries with fresh IV index and remove entries with old IV index.
If the entries with old IV index are mixed with other entries, this will
cause fragmentation of `replay_list`. The next time `bt_mesh_rpl_check`
is called, it may stop iterating `replay_list` earlier than it should
because it will meet an empty entry before it iterates over all entries
in the list.

This commit does defragmentatino of `replay_list` on every
`bt_mesh_rpl_reset` by shiting existing entries to the vacated places.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>