xfs: pass xfs_buf lookup flags to xfs_*read_agi

Allow callers to pass buffer lookup flags to xfs_read_agi and
xfs_ialloc_read_agi.  This will be used in the next patch to fix a
deadlock in the online fsck inode scanner.

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