After commit 853cd8723494 it became invalid for kinst_invop() to return
0: dtrace_invop_start() would convert this to a sentinel value
indicating that it did not consume the breakpoint, and so we'd just
call kdb_trap() to handle it.

Change kinst_invop() to return NOP_INSTR after handling a matching
breakpoint. NOP_INSTR is handled by advancing the ELR, so we have to
compensate by subtracting INSTR_SIZE before returning.

Fixes: 853cd8723494 ("arm64: Clean up usage of the dtrace invop handler")