xfs: improve xfs_inobt_get_rec prototype

Most callers of xfs_inobt_get_rec need to fill a xfs_inobt_rec_incore_t, and
those who don't yet are fine with a xfs_inobt_rec_incore_t, instead of the
three individual variables, too.  So just change xfs_inobt_get_rec to write
the output into a xfs_inobt_rec_incore_t directly.

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