xfs: indentation fix in xfs_btree_get_iroot()

The indentation in this function is different from the other functions.
Those spacebars are converted to tabs to improve readability.

Signed-off-by: Kaho Ng <ngkaho1234@gmail.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>