signal: use scoped user access for old-style sigaction

The old sigaction() system call and its compat counterpart do one
access_ok() on the user structure and then a series of __get_user() or
__put_user() calls on its fields.  That was an optimization back when
each of those was a single memory access.  These days each of them
enables and disables user access on its own (stac/clac on x86), so the
up-front check saves nothing.

Convert them to scoped_user_{read,write}_access() with
unsafe_{get,put}_user(), which does the range check once and puts a
single stac/clac pair around plain moves.

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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