Remove swblk_t.

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