staging: gasket: sysfs: clean up state if ENOMEM removing mapping

If kcalloc() returns NULL in put_mapping(), continue to clean up state,
including dropping the reference on the struct device and free attribute
memory.

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