generic: test adding filesystem-level fscrypt key via key_id

generic: test adding filesystem-level fscrypt key via key_id

Add a test which tests adding a key to a filesystem's fscrypt keyring
via an "fscrypt-provisioning" keyring key.  This is an alternative to
the normal method where the raw key is given directly.

For more details, see kernel commit 93edd392cad7 ("fscrypt: support
passing a keyring key to FS_IOC_ADD_ENCRYPTION_KEY").

This test depends on an xfs_io patch which adds the '-k' option to the
'add_enckey' command, e.g.:

	xfs_io -c "add_enckey -k KEY_ID" MOUNTPOINT

This test is skipped if the needed kernel or xfs_io support is absent.

This has been tested on ext4, f2fs, and ubifs.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>