libzfs_sendrecv: Style pass on dump_snapshot

• Add a high level comment.
• Avoid unnecessary line wrapping.
• Simplify size accounting logic.
• Eliminate unnecessary buffer on the stack.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #12967