namespaces: move proc_net_get_sb to a generic fs/super.c helper

The mqueuefs filesystem will use this helper as well.  Proc's main get_sb
could also be made to use it, but that will require a bit more rework.

Signed-off-by: Serge E. Hallyn <serue@us.ibm.com>
Cc: Cedric Le Goater <clg@fr.ibm.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>