ext4: fix ext4_init_block_bitmap() for metablock block group

When meta_bg feature is enabled and s_first_meta_bg != 0, 
ext4_init_block_bitmap() miscalculates the number of block used by
the group descriptor table (0 or 1 for metablock block group)

This patch fixes this by using ext4_bg_num_gdb()

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Stephen Tweedie <sct@redhat.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Acked-by: Andreas Dilger <adilger@sun.com>