Revert r270963. The plan is that support for asynchronous paging
is opt-in, not opt-out.
- Make default implementation of VOP_GETPAGES_ASYNC() a wrapper around
  VOP_GETPAGES(). This emulates asynchronous operation on all filesystems,
  that use their own vop_getpages(), namely: nfs, smbfs, fuse, zfs.
- Opt-in UFS to use true asynchronous vop_stdgetpages_async().

Note: other filesystems, that use default pager, need also to opt-in
after some testing.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.