make proc_ns_operations work with struct ns_common * instead of void *

We can do that now.  And kill ->inum(), while we are at it - all instances
are identical.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --cc fs/namespace.c
Simple merge

diff --cc fs/proc/inode.c
Simple merge

diff --cc fs/proc/namespaces.c
Simple merge

diff --cc include/linux/proc_ns.h
Simple merge

diff --cc ipc/namespace.c
Simple merge

diff --cc kernel/pid_namespace.c
Simple merge

diff --cc kernel/user_namespace.c
Simple merge

diff --cc kernel/utsname.c
Simple merge

diff --cc net/core/net_namespace.c
Simple merge