bury struct proc_ns in fs/proc

a) make get_proc_ns() return a pointer to struct ns_common
b) mirror ns_ops in dentry->d_fsdata of ns dentries, so that
is_mnt_ns_file() could get away with fewer dereferences.

That way struct proc_ns becomes invisible outside of fs/proc/*.c

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