Clean up code in dsl_scan_visitbp() by removing an unnecessary
alloc/free and `goto`. This has the side benefit of reducing CPU usage,
which is only really noticeable if we are not doing i/o for the leaf
blocks, like when `zfs_no_scrub_io` is set.
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Mark Maybee <mark.maybee@delphix.com>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes#15549
67894a597f