Remove dependency on linear ABD

Wherever possible it's best to avoid depending on a linear ABD.
Update the code accordingly in the following areas.

- vdev_raidz
- zio, zio_checksum
- zfs_fm
- change abd_alloc_for_io() to use abd_alloc()

Reviewed-by: David Quigley <david.quigley@intel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Gvozden Neskovic <neskovic@gmail.com>
Closes #5668

module/zfs/abd.c

@@ -722,7 +722,7 @@ abd_alloc_sametype(abd_t *sabd, size_t size)
 abd_t *
 abd_alloc_for_io(size_t size, boolean_t is_metadata)
 {
-	return (abd_alloc_linear(size, is_metadata));
+	return (abd_alloc(size, is_metadata));
 }
 
 /*