eCryptfs: Return error when lower file pointer is NULL

When an eCryptfs inode's lower file has been closed, and the pointer has
been set to NULL, return an error when trying to do a lower read or
write rather than calling BUG().

https://bugzilla.kernel.org/show_bug.cgi?id=37292



Signed-off-by: Tyler Hicks <tyhicks@linux.vnet.ibm.com>
Cc: <stable@kernel.org>