ext4: simplify parameters of add_new_gdb()

add_new_gdb() only needs the block group number; there is no need to
pass a pointer to struct ext4_new_group_data to add_new_gdb().
Instead of filling in a pointer the struct buffer_head in
add_new_gdb(), it's simpler to have the caller fetch it from the
s_group_desc[] array.

[Fixed error path to handle the case where struct buffer_head *primary
 hasn't been set yet. -- Ted]

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