xfs: store an agbno in struct xfs_imap

xfs: store an agbno in struct xfs_imap

The xfs_imap structure is embedded into the xfs_inode, which means the
size of it directly affects the inode size.  Replacing the xfs_daddr_t
with an xfs_agbno_t and taking the AG information from other easily
available sources allows us to shrink the structure including the
typical padding from 16 bytes to 8 bytes.

As a side-effect the debugging check in xfs_imap() naturally now
converges to a stricter variant that checks that the cluster is located
inside a single AG, and not just inside the entire device.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>