unnecessary alloc/free in dsl_scan_visitbp()

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