[4.19] Security fixes (blktrace, btrfs)

Here are backports of some fixes to the 4.19 stable branch.

I tested the blktrace fix with the script referenced in the commit message.

I tested the btrfs changes with the reproducers for CVE-2019-19039, CVE-2019-19377, and CVE-2019-19816, and checked for regressions with xfstests.

Ben.