MFC 230782,237274:

MFC 230782,237274:
Refine the implementation of POSIX_FADV_NOREUSE to perform
POSIX_FADV_DONTNEED requests on the currently accessed portion of the
file on each read(2) or write(2) rather than using direct I/O.  This
gives much better performance including read-ahead and write clustering
similar to normal read(2) and write(2) calls.

If subsequent read(2) and write(2) calls are sequential, then the
POSIX_FADV_DONTNEED requests will cover the entire sequentially-accessed
range.

git-svn-id: svn://svn.freebsd.org/base/stable/9@239787 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f