[XFS] move linux/log2.h header to xfs_linux.h

Generally we try not to directly include linux header files in core xfs
code; xfs_linux.h is the spot for that.

SGI-PV: 968563
SGI-Modid: xfs-linux-melb:xfs-kern:29326a

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