Fix logic error from r259821.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
Fix xfsize() for a case when file ends exactly at page boundary.
sbready() function tells that 'count' mbufs starting from the 'm'
in the buffer 'sb' are now ready.

Function returns 0 if 'm' was the first one, and now socket has data
to send, otherwise EWOULDBLOCK.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
Improve vnode_if.awk so that function pointers can be passed as VOP_*
arguments.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.