arm64: stacktrace: Report when we reach the end of the stack

Currently the arm64 unwinder code returns -EINVAL whenever it can't find
the next stack frame, not distinguishing between cases where the stack has
been corrupted or is otherwise in a state it shouldn't be and cases
where we have reached the end of the stack. At the minute none of the
callers care what error code is returned but this will be important for
reliable stack trace which needs to be sure that the stack is intact.

Change to return -ENOENT in the case where we reach the bottom of the
stack. The error codes from this function are only used in kernel, this
particular code is chosen as we are indicating that we know there is no
frame there.

Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20210224165037.24138-1-broonie@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>