New tunable parameters

While the new tunables are not intended for day-to-day tweak, it is the question of parity with Illumos - if the need to tinker with these variable ever arise the Linux way of doing so is module parameter.



Commit 55d85d5a8c45c4559a4a0e675c37b0c3afb19c2f (backport of the upstream changes)
replaced three hardcoded constants:

    #define SYNC_PASS_DEFERRED_FREE 2 /* defer frees after this pass */
    #define SYNC_PASS_DONT_COMPRESS 4 /* don't compress after this pass */
    #define SYNC_PASS_REWRITE       1 /* rewrite new bps after this pass */

with a tunable parameters:

    int zfs_sync_pass_deferred_free = 2; /* defer frees starting in this pass */
    int zfs_sync_pass_dont_compress = 5; /* don't compress starting in this pass */
    int zfs_sync_pass_rewrite = 2;       /* rewrite new bps starting in this pass */

This commit makes these tunables available as module parameters in Linux.