mips/tcg: Call probe_write() for CONFIG_USER_ONLY as well

Let's call it also for CONFIG_USER_ONLY. While at it, add a FIXME and get
rid of one local variable.

MIPS code probably needs a bigger refactoring in regards of
ensure_writable_pages(), similar to s390x, so for example, watchpoints
can be handled reliably later. The actually accessed addresses should
be probed only, not full pages.

Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Message-Id: <20190826075112.25637-6-david@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>