arm64: stacktrace: fix comment

arm64: stacktrace: fix comment

Due to a copy-paste error, we describe struct stackframe::pc as a
snapshot of the `fp` field rather than the `lr` field.

Fix the comment.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210802164845.45506-2-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>