xfs: factor out inode initialisation

Factor out code to initialize new inode clusters into a function of it's own.
This keeps xfs_ialloc_ag_alloc smaller and better structured and enables a
future inode cluster initialization transaction.  Also initialize the agno
variable earlier in xfs_ialloc_ag_alloc to avoid repeated byte swaps.

[hch:  The original patch is from Dave from his unpublished inode create
 transaction patch series, with some modifcations by me to apply stand-alone]

Signed-off-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>
Signed-off-by: Felix Blyakher <felixb@sgi.com>