xfs: pass the fsbno to xfs_perag_intent_get

All callers of xfs_perag_intent_get have a fsbno and need boilerplate
code to turn that into an agno.  Just pass the fsbno to
xfs_perag_intent_get and look up the agno there.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>