[XFS] Remove m_nreadaheads

m_nreadaheads in the mount struct is never used; remove it and the various
macros assigned to it. Also remove a couple other unused macros in the
same areas.

Removes one user of xfs_physmem.

SGI-PV: 968563
SGI-Modid: xfs-linux-melb:xfs-kern:29322a

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>