get rid of magic in proc_namespace.c

don't rely on proc_mounts->m being the first field; container_of()
is there for purpose.  No need to bother with ->private, while
we are at it - the same container_of will do nicely.

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

diff --cc fs/mount.h
Simple merge

diff --cc fs/namespace.c
Simple merge

diff --cc fs/proc_namespace.c
Simple merge