arm64: signal: don't force known signals to SIGKILL

Since commit:

  a7e6f1ca ("arm64: signal: Force SIGKILL for unknown signals in force_signal_inject")

... any signal which is not SIGKILL will be upgraded to a SIGKILL be
force_signal_inject(). This includes signals we do expect, such as
SIGILL triggered by do_undefinstr().

Fix the check to use a logical AND rather than a logical OR, permitting
signals whose layout is SIL_FAULT.

Fixes: a7e6f1ca

 ("arm64: signal: Force SIGKILL for unknown signals in force_signal_inject")
Cc: Will Deacon <will.deacon@arm.com>
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>