xfs: Remove redundant error variable from xfs_growfs_data_private()

Commit eab4e633 "xfs: uncached buffer reads need to return an error".

Remove redundant error variable, using the function level error variable
to store bp->b_error instead.

Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>