xfs_io/encrypt: support passing a keyring key to add_enckey

xfs_io/encrypt: support passing a keyring key to add_enckey

Add a '-k' option to the 'add_enckey' xfs_io command to allow exercising
the key_id field that is being added to struct fscrypt_add_key_arg.

This is needed for the corresponding test in xfstests.

For more details, see the corresponding xfstests patches as well as
kernel commit 93edd392cad7 ("fscrypt: support passing a keyring key to
FS_IOC_ADD_ENCRYPTION_KEY").

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>