Improve extents verification logic

Add functionality for extents validation inside the filesystem
extents block. The main logic is implemented under
ext4_validate_extent_entries() function, which verifies extents
or extents indexes depending of extent depth value.

PR: 259112
Reported by: Robert Morris
Reviewed by: pfg
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D33375