arm64/kprobes: Tidy up sign-extension usage

arm64/kprobes: Tidy up sign-extension usage

Kprobes does not need its own homebrewed (and frankly inscrutable) sign
extension macro; just use the standard kernel functions instead. Since
the compiler actually recognises the sign-extension idiom of the latter,
we also get the small bonus of some nicer codegen, as each displacement
calculation helper then compiles to a single optimal SBFX instruction.

Signed-off-by: Robin Murphy <[email protected]>
Acked-by: Will Deacon <[email protected]>
Signed-off-by: Catalin Marinas <[email protected]>