ecryptfs: replace BUG_ON with error handling code

commit 2c2a7552 upstream.

In crypt_scatterlist, if the crypt_stat argument is not set up
correctly, the kernel crashes. Instead, by returning an error code
upstream, the error is handled safely.

The issue is detected via a static analysis tool written by us.

Fixes: 237fead6 (ecryptfs: fs/Makefile and fs/Kconfig)
Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: Tyler Hicks <code@tyhicks.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>