arm64: compat for clock_adjtime(2) is miswired

struct timex is different on arm and arm64; adjtimex(2) takes care to
convert, clock_adjtime(2) doesn't...

Signed-off-by: Al Viro <viro@ZenIV.linux.org.uk>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: <stable@vger.kernel.org>