x86/signal: Use scoped user access in the sigreturn system calls

All four sigreturn variants (32-bit sigreturn and rt_sigreturn, native
64-bit and x32 rt_sigreturn) do an access_ok() on the whole signal frame
only to read the signal mask with __get_user().  The 64-bit and x32
variants also read uc_flags that way.  Everything else in the frame is
read by helpers that do their own checking.

Read those fields inside a scoped_user_read_access() instead, which does
the range check once and puts a single stac/clac pair around plain
loads.  The scoped accessor shadows 'frame' with a checked copy only
inside its block, so the badframe path still reports the original
pointer.

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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