linux-user: Unlock mmap_lock when resuming guest from page_unprotect

The page_unprotect() function is running everything locked. Before every
potential exit path of the function mmap_unlock() gets called to make sure
we don't leak the lock.

However, the function calls tb_invalidate_phys_page() which again can
exit a signal through longjmp, leaving our mmap_unlock() attempts in vain.

Add a hint to tb_invalidate_phys_page() that we need to unlock before we
can leave back into guest context, so that we don't leak the lock.

This fixes 16-bit i386 wine programs running in linux-user for me.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
