Add zfs_btree_verify_intensity kernel module parameter

I see a few issues in the issue tracker that might be aided by being
able to turn this on. We have no module parameter for it, so I would
like to add one.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #13874