libzfs_sendrecv: Refactor find_redact_book

Factor out get_bookmarks, find_redact_pair, and get_redact_complete
helper functions to improve the readability of find_redact_book.

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