ext4: fix on-line resizing regression

https://bugzilla.kernel.org/show_bug.cgi?id=25352

This regression was caused by commit a31437b85: "ext4: use
sb_issue_zeroout in setup_new_group_blocks", by accidentally dropping
the code which reserved the block group descriptor and inode table
blocks.

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