ext4: fix reserved gdt blocks handling in fsmap

ext4: fix reserved gdt blocks handling in fsmap

In some cases like small FSes with no meta_bg and where the resize
doesn't need extra gdt blocks as it can fit in the current one,
s_reserved_gdt_blocks is set as 0, which causes fsmap to emit a 0
length entry, which is incorrect.

  $ mkfs.ext4 -b 65536 -O bigalloc /dev/sda 5G
  $ mount /dev/sda /mnt/scratch
  $ xfs_io -c "fsmap -d" /mnt/scartch

        0: 253:48 [0..127]: static fs metadata 128
        1: 253:48 [128..255]: special 102:1 128
        2: 253:48 [256..255]: special 102:2 0     <---- 0 len entry
        3: 253:48 [256..383]: special 102:3 128

Fix this by adding a check for this case.

Cc: stable@kernel.org
Fixes: 0c9ec4beecac ("ext4: support GETFSMAP ioctls")
Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Link: https://patch.msgid.link/08781b796453a5770112aa96ad14c864fbf31935.1754377641.git.ojaswin@linux.ibm.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>