net: if: Rework interface locking

Instead of having a single global mutex, covering, in most cases
unrelated, access to all network interfaces, introduce a separate mutex
for each network interface.

Wherever interface function operates on an individual interface context
data, use local mutex protection instead. The global mutex remains only
to protect access to shared resources (IPv4/6 context pool, timer lists
etc.).

Finally, rework the timer expiry handlers, to prevent interface
functions from being called while the global lock is held (the global
mutex has to be held when iterating over the timer list, as lists are
not thread safe). The reason for this is to avoid possible deadlock
scenario. The global mutex is sometimes being locked, while inside
interface function (protected by a local mutex), so if it was allowed
the other way as well, a deadlock could occur.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>