xfs: remove the alloc_done argument to xfs_dialloc

We can simplify check the IO_agbp pointer for being non-NULL instead of
passing another argument through two layers of function calls.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>