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

cmd/raidz_test/raidz_test.c

@@ -492,8 +492,9 @@ vdev_raidz_map_alloc_expanded(abd_t *abd, uint64_t size, uint64_t offset,
 					    (dc - r) * (rows - 1) + row;
 				}
 				rr->rr_col[c].rc_size = 1ULL << ashift;
-				rr->rr_col[c].rc_abd =
-				    abd_get_offset(abd, off << ashift);
+				rr->rr_col[c].rc_abd = abd_get_offset_struct(
+				    &rr->rr_col[c].rc_abdstruct,
+				    abd, off << ashift, 1 << ashift);
 			}
 
 			asize += rr->rr_col[c].rc_size;