staging: gasket: Return EBUSY on mapping create when already in use

gasket_sysfs_create_mapping() return EBUSY if sysfs mapping already in
use, as a more appropriate error code than the current return of EINVAL,
which would indicate invalid parameters.

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