xfs/139 with parent pointers enabled occasionally pops up a corruption
message when online fsck force-rebuild repairs an AGFL:

 XFS (sde): Metadata corruption detected at xfs_agf_verify+0x11e/0x220 [xfs], xfs_agf block 0x9e0001
 XFS (sde): Unmount and run xfs_repair
 XFS (sde): First 128 bytes of corrupted metadata buffer:
 00000000: 58 41 47 46 00 00 00 01 00 00 00 4f 00 00 40 00  XAGF.......O..@.
 00000010: 00 00 00 01 00 00 00 02 00 00 00 05 00 00 00 01  ................
 00000020: 00 00 00 01 00 00 00 01 00 00 00 00 ff ff ff ff  ................
 00000030: 00 00 00 00 00 00 00 05 00 00 00 05 00 00 00 00  ................
 00000040: 91 2e 6f b1 ed 61 4b 4d 8c 9b 6e 87 08 bb f6 36  ..o..aKM..n....6
 00000050: 00 00 00 01 00 00 00 01 00 00 00 06 00 00 00 01  ................
 00000060: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
 00000070: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................

The root cause of this failure is that prior to the repair, there were
zero blocks in the AGFL.  This scenario is set up by the test case, since
it formats with 64MB AGs and tries to ENOSPC the whole filesystem.  In
this case of flcount==0, we reset fllast to -1U, which then trips the
write verifier's check that fllast is less than xfs_agfl_size().

Correct this code to set fllast to the last possible slot in the AGFL
when flcount is zero, which mirrors the behavior of xfs_repair phase5
when it has to create a totally empty AGFL.

Fixes: 0e93d3f4 ("xfs: repair the AGFL")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>