ecryptfs: fix kernel panic with null dev_name

When mounting eCryptfs, a null "dev_name" argument to ecryptfs_mount()
causes a kernel panic if the parsed options are valid. The easiest way to
reproduce this is to call mount() from userspace with an existing
eCryptfs mount's options and a "source" argument of 0.

Error out if "dev_name" is null in ecryptfs_mount()

Fixes: 237fead6

 ("[PATCH] ecryptfs: fs/Makefile and fs/Kconfig")
Cc: stable@vger.kernel.org
Signed-off-by: Jeffrey Mitchell <jeffrey.mitchell@starlab.io>
Signed-off-by: Tyler Hicks <code@tyhicks.com>