MFC revision 197579 and 199617:

MFC revision 197579 and 199617:

Add two new fcntls to enable/disable read-ahead:

 - F_READAHEAD: specify the amount for sequential access.  The amount is
   specified in bytes and is rounded up to nearest block size.
 - F_RDAHEAD: Darwin compatible version that use 128KB as the sequential
   access size.

A third argument of zero disables the read-ahead behavior.

Please note that the read-ahead amount is also constrainted by sysctl
variable, vfs.read_max, which may need to be raised in order to better
utilize this feature.

Thanks Igor Sysoev for proposing the feature and submitting the original
version, and kib@ for his valuable comments.

git-svn-id: svn://svn.freebsd.org/base/stable/8@200229 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f