kselftest/arm64: Only validate each signal context once

kselftest/arm64: Only validate each signal context once

Currently for the more complex signal context types we validate the context
specific details the end of the parsing loop validate_reserved() if we've
ever seen a context of that type. This is currently merely a bit inefficient
but will get a bit awkward when we start parsing extra_context, at which
point we will need to reset the head to advance into the extra space that
extra_context provides. Instead only do the more detailed checks on each
context type the first time we see that context type.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220829160703.874492-7-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>