asm-generic: make get_user() clear the destination on errors

[ Upstream commit 9ad18b75c2f6e4a78ce204e79f37781f8815c0fa ]

both for access_ok() failures and for faults halfway through

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>