Use bogus_page to properly reduce number of I/Os in sendfile(2). The new
sendfile_swapin() loop works this way:

• Find first invalid page in the request.
• Do vm_pager_has_page() and get count of pages, that can be taken in single I/O.
• Trim valid pages from the end of the request.
• Cycle through the request and substitute to bogus_page all valid pages that are in the middle of the request.
• After I/O launched (pager copies array of pages into buf(9), it is important to restore proper page pointers with help vm_page_lookup().

Count bogus pages used and report them in sendfile stats.