xfs: Fix the owner setting issue for rmap query in xfs fsmap

xfs: Fix the owner setting issue for rmap query in xfs fsmap

I notice a rmap query bug in xfs_io fsmap:
[root@fedora ~]# xfs_io -c 'fsmap -vvvv' /mnt
 EXT: DEV    BLOCK-RANGE           OWNER              FILE-OFFSET      AG AG-OFFSET             TOTAL
   0: 253:16 [0..7]:               static fs metadata                  0  (0..7)                    8
   1: 253:16 [8..23]:              per-AG metadata                     0  (8..23)                  16
   2: 253:16 [24..39]:             inode btree                         0  (24..39)                 16
   3: 253:16 [40..47]:             per-AG metadata                     0  (40..47)                  8
   4: 253:16 [48..55]:             refcount btree                      0  (48..55)                  8
   5: 253:16 [56..103]:            per-AG metadata                     0  (56..103)                48
   6: 253:16 [104..127]:           free space                          0  (104..127)               24
   ......

Bug:
[root@fedora ~]# xfs_io -c 'fsmap -vvvv -d 0 3' /mnt
[root@fedora ~]#
Normally, we should be able to get one record, but we got nothing.

The root cause of this problem lies in the incorrect setting of rm_owner in
the rmap query. In the case of the initial query where the owner is not
set, __xfs_getfsmap_datadev() first sets info->high.rm_owner to ULLONG_MAX.
This is done to prevent any omissions when comparing rmap items. However,
if the current ag is detected to be the last one, the function sets info's
high_irec based on the provided key. If high->rm_owner is not specified, it
should continue to be set to ULLONG_MAX; otherwise, there will be issues
with interval omissions. For example, consider "start" and "end" within the
same block. If high->rm_owner == 0, it will be smaller than the founded
record in rmapbt, resulting in a query with no records. The main call stack
is as follows:

xfs_ioc_getfsmap
  xfs_getfsmap
    xfs_getfsmap_datadev_rmapbt
      __xfs_getfsmap_datadev
        info->high.rm_owner = ULLONG_MAX
        if (pag->pag_agno == end_ag)
	  xfs_fsmap_owner_to_rmap
	    // set info->high.rm_owner = 0 because fmr_owner == -1ULL
	    dest->rm_owner = 0
	// get nothing
	xfs_getfsmap_datadev_rmapbt_query

The problem can be resolved by simply modify the xfs_fsmap_owner_to_rmap
function internal logic to achieve.

After applying this patch, the above problem have been solved:
[root@fedora ~]# xfs_io -c 'fsmap -vvvv -d 0 3' /mnt
 EXT: DEV    BLOCK-RANGE      OWNER              FILE-OFFSET      AG AG-OFFSET        TOTAL
   0: 253:16 [0..7]:          static fs metadata                  0  (0..7)               8

Fixes: e89c041338ed ("xfs: implement the GETFSMAP ioctl")
Signed-off-by: Zizhi Wo <wozizhi@huawei.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>