ext4: simplify the gdbblock calculation in add_new_gdb_meta_bg

We always call add_new_gdb_meta_bg with first group in mete_bg. Remove the
unnecessary ext4_meta_bg_first_group conversion to simplify the gdbblock
calculation.

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Link: https://lore.kernel.org/r/20230826174712.4059355-12-shikemeng@huaweicloud.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>