linux-user: assert that target_mprotect cannot fail

All error conditions that target_mprotect checks are also checked
by target_mmap.  EACCESS cannot happen because we are just removing
PROT_WRITE.  ENOMEM should not happen because we are modifying a
whole VMA (and we have bigger problems anyway if it happens).

Fixes a Coverity false positive, where Coverity complains about
target_mprotect's return value being passed to tb_invalidate_phys_range.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>

linux-user/mmap.c

@@ -514,10 +514,7 @@ abi_long target_mmap(abi_ulong start, abi_ulong len, int prot,
                 goto fail;
             if (!(prot & PROT_WRITE)) {
                 ret = target_mprotect(start, len, prot);
-                if (ret != 0) {
-                    start = ret;
-                    goto the_end;
-                }
+                assert(ret == 0);
             }
             goto the_end;
         }