xfs_io: implement 'set_encpolicy' and 'get_encpolicy' commands

xfs_io: implement 'set_encpolicy' and 'get_encpolicy' commands

Add set_encpolicy and get_encpolicy commands to xfs_io so that xfstests
will be able to test filesystem encryption using the actual user API,
not just hacked in with a mount option.  These commands use the common
"fscrypt" API currently implemented by ext4 and f2fs, but it's also
under development for ubifs and planned for xfs.

Note that to get encrypted files to actually work, it's also necessary
to add a key to the kernel keyring.  This patch does not add a command
for this to xfs_io because it's possible to do it using keyctl.  keyctl
can also be used to remove keys, revoke keys, invalidate keys, etc.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>