bitops.h: sextract64() return type should be int64_t, not uint64_t

The documentation for sextract64() claims that the return type is
an int64_t, but the code itself disagrees. Fix the return type to
conform to the documentation and to bring it into line with
sextract32(), which returns int32_t.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NRichard Henderson <rth@twiddle.net>
Message-id: 1423231328-15662-1-git-send-email-peter.maydell@linaro.org

include/qemu/bitops.h

@@ -354,7 +354,7 @@ static inline int32_t sextract32(uint32_t value, int start, int length)
  * Returns: the sign extended value of the bit field extracted from the
  * input value.
  */
-static inline uint64_t sextract64(uint64_t value, int start, int length)
+static inline int64_t sextract64(uint64_t value, int start, int length)
 {
     assert(start >= 0 && length > 0 && length <= 64 - start);
     /* Note that this implementation relies on right shift of signed