selinux: unify OOM handling in network hashtables

For network objects, like interfaces, nodes, port and InfiniBands, the
object to SID lookup is cached in hashtables.  OOM during such hashtable
additions of new objects is considered non-fatal and the computed SID is
simply returned without adding the compute result into the hash table.

Actually ignore OOM in the InfiniBand code, despite the comment already
suggesting to do so.  This reverts commit c350f8be ("selinux: Fix
error return code in sel_ib_pkey_sid_slow()").

Add comments in the other places.

Use kmalloc() instead of kzalloc(), since all members are initialized on
success and the data is only used in internbal hash tables, so no risk
of information leakage to userspace.

Fixes: c350f8be ("selinux: Fix error return code in sel_ib_pkey_sid_slow()")
Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>