xfs: rename the next_agno perag iteration variable

Rename the next_agno variable to be consistent across the several
iteration macros and shorten line length.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>