sys/linux: add some basic fscrypt test programs

With fscrypt (a.k.a. ext4, f2fs, and ubifs encryption), several steps
are required to get to a properly set-up encrypted directory:

   - Mount a filesystem that supports encryption.
   - Call FS_IOC_ADD_ENCRYPTION_KEY on the mountpoint.
   - Create a directory in the filesystem.
   - Call FS_IOC_SET_ENCRYPTION_POLICY on the directory, specifying the
     encryption key that was added earlier.
   - Then actually create stuff in the directory.

It may be too hard for syzkaller to generate all these steps, which may
explain why syzbot isn't yet properly covering the fscrypt code; see
https://storage.googleapis.com/syzkaller/cover/ci-upstream-kasan-gce.html
which currently shows only 5% coverage of the fs/crypto/ directory.

Therefore add some test programs which do this setup on ext4 and f2fs.

Note that ubifs support can't be included yet because syzkaller doesn't
yet know how to mount an ubifs filesystem (which would likely require
using block2mtd, as ubifs uses MTD devices rather than block devices).

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