* userspace: aligned buffers. Minimum of 32B alignment is
needed for AVX2. Kernel buffers are aligned 512B or more.
* add abd_get_offset_size() interface
* abd_iter_map(): fix calculation of iter_mapsize
* add abd_raidz_gen_iterate() and abd_raidz_rec_iterate()
Signed-off-by: Gvozden Neskovic <neskovic@gmail.com>