alpha: use scoped user access when setting up signal frames

setup_sigcontext() writes the whole register set with __put_user() and
__copy_to_user(), OR-ing the errors together, relying on its callers'
access_ok() on the frame.  setup_frame() and setup_rt_frame() write the
sigreturn trampoline, and setup_rt_frame() the ucontext header, the
same way.

Do those writes inside scoped_user_write_access() with
unsafe_put_user() and unsafe_copy_to_user().  on_sig_stack() is
evaluated before the scope, and the instruction cache flush stays
outside it.  The up-front access_ok()
calls stay: setup_rt_frame() still needs its own for the
__copy_to_user() of the signal mask.

A fault part way through now stops at the first failing write instead of
carrying on.  Either way the frame setup fails and the task gets
SIGSEGV.  alpha 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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