xfs: replace xfs_ag_block_count() with perag accesses

Many of the places that call xfs_ag_block_count() have a perag
available. These places can just read pag->block_count directly
instead of calculating the AG block count from first principles.

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