ashmem: fix a typo introduced by Samsung (DerTeufel)

Including Google's official mach-msm Linux kernel,
lots of Android kernels are returning -1 here.

It should return -1 because it's the appropriate value
when locking mutex has failed.

Also, the initial ashmem writer, Robert Love also committed code returning -1.

We predict that Samsung has made this typo because
the upper code, "if (!sc->nr_to_scan)" returns lru_count.

Credit goes to DerTeufel for finding this typo.
sync: don't block the flusher thread waiting on IO