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

man/man4/zfs.4

@@ -1354,6 +1354,22 @@ _
 .TE
 .Sy \& * No Requires debug build.
 .
+.It Sy zfs_btree_verify_intensity Ns = Ns Sy 0 Pq uint
+Enables btree verification.
+The following settings are culminative:
+.TS
+box;
+lbz r l l .
+	Value	Description
+
+	1	Verify height.
+	2	Verify pointers from children to parent.
+	3	Verify element counts.
+	4	Verify element order. (expensive)
+*	5	Verify unused memory is poisoned. (expensive)
+.TE
+.Sy \& * No Requires debug build.
+.
 .It Sy zfs_free_leak_on_eio Ns = Ns Sy 0 Ns | Ns 1 Pq int
 If destroy encounters an
 .Sy EIO