Externalize nsw_cluster_max and initialize it early.

GEOM_ELI needs to know the value, cause it will soon have special
memory handling for IO operations associated with swap.

Move initialization to swap_pager_init(), which is executed at
SI_SUB_VM, unlike swap_pager_swap_init(), which would be executed
only when a swap is configured. GEOM_ELI might need the value at
SI_SUB_DRIVERS, when disks are tasted by GEOM.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D24400