Respect the caller's hints when performing swap readahead.

The pager getpages interface allows the caller to bound the number of
readahead and readbehind pages, and vm_fault_hold() makes use of this
feature. These bounds were ignored after r305056, causing the swap pager
to potentially page in more than the specified number of pages.

Reported and reviewed by: alc
X-MFC with: r305056