libxfs: unify xfs_btree.c with kernel code

libxfs: unify xfs_btree.c with kernel code

The libxfs/xfs_btree.c code does not contain a small amount of code
for btree block readahead that the kernel code does. Instead, it
short circuits it at a higher layer and doesn't include the lower
layer functions. There is no harm in calling the lower lay functions
and have them do nothing, and doing so unifies the kernel and
userspace code.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>