s390: use get_user()/put_user() instead of the unchecked variants

On s390 user space is a separate address space, so access_ok() has
nothing to check, and get_user() and put_user() are __get_user() and
__put_user() plus might_fault().  The remaining callers can use the
normal accessors directly:

 - signal frame setup writes the back chain, the signal number and the
   ucontext header in process context;

 - the user stack walker for stack traces and perf callchains already
   runs under pagefault_disable(), where might_fault() returns early;

 - the general register set writer for ptrace runs in the tracer's
   process context with no locks held.

This is part of getting rid of __get_user() and __put_user() entirely.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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