ext4: pass verify_reserved_gdb() the number of group decriptors

The 64bit resizer adds a flex group each time, so verify_reserved_gdb
can not use s_groups_count directly, it should use the number of group
decriptors before the added group.

Signed-off-by: Yongqiang Yang <xiaoqiangnk@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>