xfs: pass the exact range to initialize to xfs_initialize_perag

Currently only the new agcount is passed to xfs_initialize_perag, which
requires lookups of existing AGs to skip them and complicates error
handling.  Also pass the previous agcount so that the range that
xfs_initialize_perag operates on is exactly defined.  That way the
extra lookups can be avoided, and error handling can clean up the
exact range from the old count to the last added perag structure.

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