Return -1 for generic kmem cache shrinker

It has been observed that it's possible to get in a state where
shrink_slabs() will spin repeated invoking the generic kmem cache
shrinker.  It fails to detect it's not making forward progress
reclaiming from the cache and doesn't give up.  To ensure this
never occurs we unconditionally return -1 after reclaiming what
we can.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <ryao@gentoo.org>
Closes zfsonlinux/zfs#1276
Closes zfsonlinux/zfs#1598
Closes zfsonlinux/zfs#1432
Fix KMC_OFFSLAB type caches

Because spl_slab_size() was always returning -ENOSPC for caches of
type KMC_OFFSLAB the cache could never be created.  Additionally
the slab size is rounded up to a page which is what kv_alloc()
expects.  The kv_alloc() code will minimally allocate a page,
in the KMC_OFFSLAB case this could be reduced.

The basic regression tests kmem:slab_small, kmem:slab_large,
and kmem:slab_align regression were updated to test KMC_OFFSLAB.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ying Zhu <casualfisher@gmail.com>
Closes #266