[$] Per-extent encrypted keys for fscrypt

The kernel's fscryptsubsystem enables filesystems to store files and directories in encrypted form, protecting them against offline attacks. Afew filesystems support encryption with fscrypt currently, but Btrfs is anexception, despite a number of attempts to add this feature. The problemis that, as so often seems to be the case, Btrfs works differently and doesnot fit well with one of the key assumptions in the design of fscrypt. With thispatch series, Sweet Tea Dorminy is working to enhance fscrypt to be abetter fit for filesystems like Btrfs.