staging: erofs: fix a compile warning of Z_EROFS_VLE_VMAP_ONSTACK_PAGES

There is a type mismatch in the definition of
Z_EROFS_VLE_VMAP_ONSTACK_PAGES, let's fix it.

Link: https://lists.01.org/pipermail/kbuild-all/2018-July/050707.html


Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>