netns: Remove __peernet2id_alloc()

__peernet2id_alloc() was used for both plain lookups and for netns ID
allocations (depending the value of '*alloc'). Let's separate lookups
from allocations instead. That is, integrate the lookup code into
__peernet2id() and make peernet2id_alloc() responsible for allocating
new netns IDs when necessary.

This makes it clear that __peernet2id() doesn't modify the idr and
prepares the code for lockless lookups.

Also, mark the 'net' argument of __peernet2id() as 'const', since we're
modifying this line.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>