arm64: insn: Add return statements after BUG_ON()

Following a recent series of enhancements to the insn code the ARMv8
allnoconfig build has been generating a large number of warnings in the
form of:

arch/arm64/kernel/insn.c:689:8: warning: 'insn' may be used uninitialized in this function [-Wmaybe-uninitialized]

This is because BUG() and related macros can be compiled out so we get
execution paths which normally result in a panic compiling out to noops
instead.

I wasn't able to immediately identify a sensible return value to use in
these cases so just return AARCH64_BREAK_FAULT - this is all "should
never happen" code so hopefully it never has a practical impact.

Signed-off-by: Mark Brown <broonie@kernel.org>
[catalin.marinas@arm.com: AARCH64_BREAK_FAULT definition contributed by Daniel Borkmann]
[catalin.marinas@arm.com: replace return 0 with AARCH64_BREAK_FAULT]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>