Restore swap pager readahead after r292373.

The removal of vm_fault_additional_pages() meant that a hard fault on
a swap-backed page would result in only that page being read in. This
change implements readahead and readbehind for the swap pager in
swap_pager_getpages(). swap_pager_haspage() is modified to return the
largest contiguous non-resident range of pages containing the requested
range.

Reviewed by: alc, kib
Tested by: pho
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D7677