xfs: fix agno increment in xfs_inumbers() loop

caused a regression in xfs_inumbers, which in turn broke
xfsdump, causing incomplete dumps.

The loop in xfs_inumbers() needs to fill the user-supplied
buffers, and iterates via xfs_btree_increment, reading new
ags as needed.

But the first time through the loop, if xfs_btree_increment()
succeeds, we continue, which triggers the ++agno at the bottom
of the loop, and we skip to soon to the next ag - without
the proper setup under next_ag to read the next ag.

Fix this by removing the agno increment from the loop conditional,
and only increment agno if we have actually hit the code under
the next_ag: target.

Cc: stable@vger.kernel.org
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>