nstree: check listing permission before taking a namespace reference

In the Linux kernel, the following vulnerability has been resolved:

nstree: check listing permission before taking a namespace reference

legitimize_ns() takes a reference on the candidate namespace before
may_list_ns() has decided whether the caller may see it. The
__free(ns_put) cleanup on the denied path can drop the last reference to a
mount namespace while we still hold the rcu read lock, and put_mnt_ns()
may sleep there. This is the same problem commit 2ec2aff3c8e2 ("ns: make
sure reference are dropped outside of rcu lock") fixed for the put_user()
path. Neither ns_requested() nor may_list_ns() needs a reference, both
only look at the namespace type and at the caller's own namespaces, so do
the checks first and take the reference last.

Splat:

Voluntary context switch within RCU read-side critical section!
WARNING: kernel/rcu/tree_plugin.h:332 at rcu_note_context_switch+0x238/0x2a0, CPU#5: a/3442
CPU: 5 UID: 1000 PID: 3442 Comm: a Not tainted 7.0.0-30-generic #30-Ubuntu PREEMPT(lazy)
RIP: 0010:rcu_note_context_switch+0x238/0x2a0
Call Trace:
<TASK>
__schedule+0xcf/0x650
schedule+0x27/0x90
schedule_preempt_disabled+0x15/0x30
__mutex_lock.constprop.0+0x550/0xaf0
__mutex_lock_slowpath+0x13/0x20
mutex_lock+0x3b/0x50
exp_funnel_lock+0xb2/0x260
synchronize_rcu_expedited+0xe7/0x220
namespace_unlock+0x26a/0x320
put_mnt_ns+0xd3/0x120
mntns_put+0xe/0x20
do_listns+0x13e/0x560
__do_sys_listns+0x126/0x2d0
__x64_sys_listns+0x20/0x30
x64_sys_call+0x2366/0x2390
do_syscall_64+0x105/0x5a0
entry_SYSCALL_64_after_hwframe+0x76/0x7e
</TASK>