ucount: Make get_ucount a safe get_user replacement

When the ucount code was refactored to create get_ucount it was missed
that some of the contexts in which a rlimit is kept elevated can be
the only reference to the user/ucount in the system.

Ordinary ucount references exist in places that also have a reference
to the user namspace, but in POSIX message queues, the SysV shm code,
and the SIGPENDING code there is no independent user namespace
reference.

Inspection of the the user_namespace show no instance of circular
references between struct ucounts and the user_namespace.  So
hold a reference from struct ucount to i's user_namespace to
resolve this problem.

Link: https://lore.kernel.org/lkml/[email protected]/
Reported-by: Qian Cai <[email protected]>
Reported-by: Mathias Krause <[email protected]>
Tested-by: Mathias Krause <[email protected]>
Reviewed-by: Mathias Krause <[email protected]>
Reviewed-by: Alexey Gladkov <[email protected]>
Fixes: d64696905554 ("Reimplement RLIMIT_SIGPENDING on top of ucounts")
Fixes: 6e52a9f0532f ("Reimplement RLIMIT_MSGQUEUE on top of ucounts")
Fixes: d7c9e99aee48 ("Reimplement RLIMIT_MEMLOCK on top of ucounts")
Cc: [email protected]
Signed-off-by: "Eric W. Biederman" <[email protected]>

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