ptrace: use plain get_user()/put_user() for the regset iovec

PTRACE_GETREGSET and PTRACE_SETREGSET do an access_ok() on the user
iovec, read its two fields with __get_user(), and once the regset has
been transferred write the length back with __put_user().  Each of those
accesses enables and disables user access on its own, so the up-front
check saves nothing.  On x86, __get_user() is even more expensive than
get_user(): it does an lfence, where get_user() checks the address with
a couple of plain instructions.

Just use get_user() and put_user(), and drop the access_ok().  Both
fields are read before anything else is done, so a bad pointer still
fails with -EFAULT up front.

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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