zfs_arc: support for zfsonlinux

zfs_arc: support for zfsonlinux

fix alocation schemes so the plugin actually works
 - specify type llist_t kstat_t, so we can easily pass in our parsed data
 - read the file into one chunk of memory so we have the values available
 - count the number of lines to allocate the long pointer array
 - parse the buffer line by line.