nios2: use scoped user access for the signal frame

rt_setup_ucontext() and rt_restore_ucontext() move the saved registers
with __put_user() and __get_user(), OR-ing the errors together, relying
on the access_ok() done on the whole frame by setup_rt_frame() and
do_rt_sigreturn().  setup_rt_frame() writes the ucontext flags and link
the same way.

Do those accesses inside scoped_user_{write,read}_access() instead, with
unsafe_{put,get}_user().  The gregs pointer is now derived inside each
scope, from the pointer the scope checked.  restore_altstack() is called
after the scope.  The access_ok() calls stay: do_rt_sigreturn() still
needs its own for the __copy_from_user() of the signal mask.

A fault part way through now stops at the first failing access instead
of carrying on.  Either way the task gets SIGSEGV.  nios2 has no
user_access_begin(), so the accesses themselves are the same as before.

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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