mips/signal: In force_fcr31_sig return in the impossible case

mips/signal: In force_fcr31_sig return in the impossible case

In a recent discussion Maciej Rozycki reported that this case is
impossible.

Handle the impossible case by just returning instead of trying to
handle it.  This makes static analysis simpler as it means nothing
needs to consider the impossible case after the return statement.

As the code no longer has to deal with this case remove FPE_FIXME from
the mips siginfo.h

Cc: "Maciej W. Rozycki" <macro@imgtec.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Link: http://lkml.kernel.org/r/20170718140651.15973-4-ebiederm@xmission.com
Ref: ea1b75cf9138 ("signal/mips: Document a conflict with SI_USER with SIGFPE")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>