linux-user/sparc/signal.c: Remove dead code

linux-user/sparc/signal.c: Remove dead code

Coverity complains (CID 1390847) about some dead code in
do_sigreturn(). This is an if (err) clause that can never be
true, copied from the kernel (where __get_user returns an error).
The one code path that could report an error is in the
currently commented-out pseudocode for handling FPU register
restoring, so move the if into that comment (and fix the
broken indent in the comment in the process).

(The new position for the error check is also the semantically
correct one -- we should not restore the signal mask from
the signal frame if we get an error here, so the check must
be done before set_sigmask(), not after.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-id: 20181115114616.26265-1-peter.maydell@linaro.org