powerpc/64s/interrupt: clean up interrupt return labels

Normal kernel-interrupt exits can get interrupt_return_srr_user_restart
in their backtrace, which is an unusual and notable function, and it is
part of the user-interrupt exit path, which is doubly confusing.

Add non-local labels for both user and kernel interrupt exit cases to
address this and make the user and kernel cases more symmetric. Also get
rid of an unused label.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210630074621.2109197-9-npiggin@gmail.com