kselftest/arm64: Fix validation of EXTRA_CONTEXT signal context location

kselftest/arm64: Fix validation of EXTRA_CONTEXT signal context location

Currently in validate_extra_context() we assert both that the extra data
pointed to by the EXTRA_CONTEXT is 16 byte aligned and that it immediately
follows the struct _aarch64_ctx providing the terminator for the linked
list of contexts in the signal frame. Since struct _aarch64_ctx is an 8
byte structure which must be 16 byte aligned these cannot both be true. As
documented in sigcontext.h and implemented by the kernel the extra data
should be at the next 16 byte aligned address after the terminator so fix
the validation to match.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220829160703.874492-5-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>