Fix off-by-one bug in range tree code

Without this fix, zfs_range_tree_find_in could return an overlap when
the found range starts immediately after the searched range, with no
actual overlap.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed by: Igor Kozhukhov <ikozhukhov@gmail.com>
Signed-off-by: Paul Dagnelie <paul.dagnelie@klarasystems.com>
Closes #17363