ext4 crypto: replace some BUG_ON()'s with error checks

commit 687c3c36 upstream.

Buggy (or hostile) userspace should not be able to cause the kernel to
crash.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>