UPSTREAM: procfs: make freeing proc_fs_info rcu-delayed

makes proc_pid_ns() safe from rcu pathwalk (put_pid_ns()
is still synchronous, but that's not a problem - it does
rcu-delay everything that needs to be)

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
(cherry picked from commit e31f0a57)
Change-Id: I7f206580dd0c26d1a60354a7797daed1026dc73c
Bug: 306055483
Signed-off-by: Todd Kjos <tkjos@google.com>