staging: gasket: sysfs: fix potential null dereference

Add handling of possible allocation failure.

Reported by smatch:

  drivers/staging/gasket/gasket_sysfs.c:105 put_mapping() error: potential null dereference 'files_to_remove'. (kcalloc returns null)

Signed-off-by: Ivan Bornyakov <brnkv.i1@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>