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