Remove unnecessary soft busy of the page before to do vn_rdwr() in
kern_sendfile() which is unnecessary.
The page is already wired so it will not be subjected to pagefault.
The content cannot be effectively protected as it is full of races
already.
Multiple accesses to the same indexes are serialized through vn_rdwr().

Sponsored by:	EMC / Isilon storage division
Reviewed by:	alc, jeff
Tested by:	pho
Fix implicit declaration of warnx().