fuse: Fix build on 32-bit architectures

MFC 3d721de049be ("Fix NFS exports of FUSE file systems for big
directories") missed a case of a uint64_t from HEAD that should be a
u_long in 13 due to KPI differences. Specifically, HEAD has b214fcceacad
("Change VOP_READDIR's cookies argument to a **uint64_t"), but stable/13
does not.

This is a direct commit to stable/13.