staging: gasket: remove unnecessary NULL checks on calls from VFS

Remove unneeded checks for NULL pointers in struct file pointers passed
from the VFS layer or the private_data that must have been properly set
at file open time.

Reported-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>