Verify BPs in spa_load_verify_cb() and dsl_scan_visitbp()

We want zpool import to be highly robust and never panic, even
when encountering corrupt metadata. This is already handled in the
arc_read() code path, which covers most cases, but spa_load_verify_cb()
relies on zio_read() and is responsible for verifying the block pointer.

During import it is also possible to encounter blocks pointers which
contain ZIO_COMPRESS_INHERIT and ZIO_CHECKSUM_INHERIT values. Relax
the verification function slightly to allow this.

Futhermore, extend dsl_scan_recurse() to verify the block pointer
contents of level zero blocks which are not of type DMU_OT_DNODE or
DMU_OT_OBJSET. This is handled by arc_read() in the other cases.

Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13124
Closes #13360