xfs: convert btree cursor ag-private member name

bc_private.a -> bc_ag conversion via script:

`sed -i 's/bc_private\.a/bc_ag/g' fs/xfs/*[ch] fs/xfs/*/*[ch]`

And then revert the change to the bc_ag #define in
fs/xfs/libxfs/xfs_btree.h manually.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>