xfs: don't update lastino for FSBULKSTAT_SINGLE

The kernel test robot found a regression of xfs/054 in the conversion of
bulkstat to use the new iwalk infrastructure -- if a caller set *lastip
= 128 and invoked FSBULKSTAT_SINGLE, the bstat info would be for inode
128, but *lastip would be increased by the kernel to 129.

FSBULKSTAT_SINGLE never incremented lastip before, so it's incorrect to
make such an update to the internal lastino value now.

Fixes: 2810bd68 ("xfs: convert bulkstat to new iwalk infrastructure")
Reported-by: kernel test robot <rong.a.chen@intel.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>