ubifs: wire up new fscrypt ioctls

Wire up the new ioctls for adding and removing fscrypt keys to/from the
filesystem, and the new ioctl for retrieving v2 encryption policies.

The key removal ioctls also required making UBIFS use
fscrypt_drop_inode().

For more details see Documentation/filesystems/fscrypt.rst and the
fscrypt patches that added the implementation of these ioctls.

Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Eric Biggers <ebiggers@google.com>