Remove swblk_t.

It was used only to store the bounds of each swap device. However,
since swblk_t is a signed 32-bit int and daddr_t is a signed 64-bit
int, swp_pager_isondev() may return an invalid result if swap devices
are repeatedly added and removed and sw_end for a device ends up
becoming a negative number.

Note that the removed comment about maximum swap size still applies.

Reviewed by: jeff, kib
Tested by: pho
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23666