xfs: remove impossible to read code in xfs_bmap_add_extent_delay_real

We already have the worst case blocks reserved, so xfs_icsb_modify_counters
won't fail in xfs_bmap_add_extent_delay_real.  In fact we've had an assert
to catch this case since day and it never triggered.  So remove the code
to try smaller reservations, and just return the error for that case in
addition to keeping the assert.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>