arm64: uprobes: Add GCS support to uretprobes

Ret probes work by changing the value in the link register at
the probe location to return to the probe rather than the calling
routine. Thus the GCS needs to be updated with this address as well.

Since its possible to insert probes at locations where the
current value of the LR doesn't match the GCS state this needs
to be detected and handled in order to maintain the existing
no-fault behavior.

Co-developed-by: Steve Capper <steve.capper@arm.com>
Signed-off-by: Steve Capper <steve.capper@arm.com>
Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
[will: Add '__force' to gcspr casts in arch_uretprobe_hijack_return_addr()]
Signed-off-by: Will Deacon <will@kernel.org>