allow callers to allocate and provide the abd_t struct

The `abd_get_offset_*()` routines create an abd_t that references
another abd_t, and doesn't allocate any pages/buffers of its own.  In
some workloads, these routines may be called frequently, to create many
abd_t's representing small pieces of a single large abd_t.  In
particular, the upcoming RAIDZ Expansion project makes heavy use of
these routines.

This commit adds the ability for the caller to allocate and provide the
abd_t struct to a variant of `abd_get_offset_*()`.  This eliminates the
cost of allocating the abd_t and performing the accounting associated
with it (`abdstat_struct_size`).  The RAIDZ/DRAID code uses this for
the `rc_abd`, which references the zio's abd.  The upcoming RAIDZ
Expansion project will leverage this infrastructure to increase
performance of reads post-expansion by around 50%.

Additionally, some of the interfaces around creating and destroying
abd_t's are cleaned up.  Most significantly, the distinction between
`abd_put()` and `abd_free()` is eliminated; all types of abd_t's are
now disposed of with `abd_free()`.

Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Issue #8853 
Closes #11439

module/zfs/zil.c

@@ -1230,7 +1230,7 @@ zil_lwb_write_done(zio_t *zio)
 	ASSERT(!BP_IS_HOLE(zio->io_bp));
 	ASSERT(BP_GET_FILL(zio->io_bp) == 0);
 
-	abd_put(zio->io_abd);
+	abd_free(zio->io_abd);
 
 	mutex_enter(&zilog->zl_lock);
 	ASSERT3S(lwb->lwb_state, ==, LWB_STATE_ISSUED);