s390x/tcg: Store only the necessary amount of doublewords for STFLE

The PoP (z14, 7-382) says:
    Doublewords to the right of the doubleword in which the
    highest-numbered facility bit is assigned for a model
    may or may not be stored.

However, stack protection in certain binaries can't deal with that.
"gzip" example code:

f1b4:       a7 08 00 03             lhi     %r0,3
f1b8:       b2 b0 f0 a0             stfle   160(%r15)
f1bc:       e3 20 f0 b2 00 90       llgc    %r2,178(%r15)
f1c2:       c0 2b 00 00 00 01       nilf    %r2,1
f1c8:       b2 4f 00 10             ear     %r1,%a0
f1cc:       b9 14 00 22             lgfr    %r2,%r2
f1d0:       eb 11 00 20 00 0d       sllg    %r1,%r1,32
f1d6:       b2 4f 00 11             ear     %r1,%a1
f1da:       d5 07 f0 b8 10 28       clc     184(8,%r15),40(%r1)
f1e0:       a7 74 00 06             jne     f1ec <file_read@@base+0x1bc>
f1e4:       eb ef f1 30 00 04       lmg     %r14,%r15,304(%r15)
f1ea:       07 fe                   br      %r14
f1ec:       c0 e5 ff ff 9d 6e       brasl   %r14,2cc8 <__stack_chk_fail@plt>

In QEMU, we currently have:
    max_bytes = 24
the code asks for (3 + 1) doublewords == 32 bytes.

If we write 32 bytes instead of only 24, and return "2 + 1" doublewords
("one less than the number of doulewords needed to contain all of the
 facility bits"), the example code detects a stack corruption.

In my opinion, the code is wrong. However, it seems to work fine on
real machines. So let's limit storing to the minimum of the requested
and the maximum doublewords.

Cc: Stefan Liebler <stli@linux.ibm.com>
Cc: Andreas Krebbel <Andreas.Krebbel@de.ibm.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>

target/s390x/misc_helper.c

@@ -678,7 +678,13 @@ uint32_t HELPER(stfle)(CPUS390XState *env, uint64_t addr)
 
     prepare_stfl();
     max_bytes = ROUND_UP(used_stfl_bytes, 8);
-    for (i = 0; i < count_bytes; ++i) {
+
+    /*
+     * The PoP says that doublewords beyond the highest-numbered facility
+     * bit may or may not be stored.  However, existing hardware appears to
+     * not store the words, and existing software depend on that.
+     */
+    for (i = 0; i < MIN(count_bytes, max_bytes); ++i) {
         cpu_stb_data_ra(env, addr + i, stfl_bytes[i], ra);
     }