hexagon: use scoped user access for the signal frame

setup_sigcontext() and restore_sigcontext() 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
sys_rt_sigreturn().  setup_rt_frame() writes the two trampoline magic
words the same way.

Do those accesses inside scoped_user_{write,read}_access() instead, with
unsafe_{put,get}_user() and unsafe_copy_{to,from}_user() for the block
of general registers.  The access_ok() calls stay, for the
__copy_to_user() and __copy_from_user() of the signal mask.

A fault part way through now stops at the first failing access instead
of carrying on.  On sigreturn this also means the general registers are
no longer zero-filled past the fault, and the user PC is not updated.
Either way the task gets SIGSEGV.  hexagon uses the asm-generic
accessors and 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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