Merge from projects/sendfile:

o Provide a new VOP_GETPAGES_ASYNC(), which works like VOP_GETPAGES(), but

doesn't sleep. It returns immediately, and will execute the I/O done handler
function that must be supplied as argument.

o Provide VOP_GETPAGES_ASYNC() for the FFS, which uses vnode_pager.
o Extend pagertab to support pgo_getpages_async method, and implement this

method for vnode_pager.

Reviewed by: kib
Tested by: pho
Sponsored by: Netflix
Sponsored by: Nginx, Inc.