swap_pager: Handle large swap_pager_reserve() requests

swap_pager: Handle large swap_pager_reserve() requests

This interface is used solely by md(4) when the MD_RESERVE flag is
specified, as in `mdconfig -a -t swap -s 1G -o reserve`.  It
pre-allocates swap blocks for the entire object.

The number of blocks to be reserved is specified as a vm_size_t, but
swp_pager_getswapspace() can allocate at most INT_MAX blocks.  vm_size_t
also seems like the incorrect type to use here it refers only to the
size of the VM object, not the size of a mapping.  So:
- change the type of "size" in swap_pager_reserve() to vm_pindex_t, and
- clamp the requested number of blocks for a single
  swp_pager_getswapspace() call to INT_MAX.

Reported by:	syzkaller
Reviewed by:	dougm, alc, kib
Sponsored by:	The FreeBSD Foundation

(cherry picked from commit 686aa9287c6b3658daa2ca0ef1917f2e70a6c07e)