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

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

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org
Signed-off-by: Theodore Ts'o <tytso@google.com>
Change-Id: I5f6c811b23cbadefb4e324c4ec0c5838815725dc