The function ext4_resize_begin() gets called from three different
places, and online resize for bigalloc file systems is disallowed from
the old-style online resize (EXT4_IOC_GROUP_ADD and
EXT4_IOC_GROUP_EXTEND), but it *is* supposed to be allowed via
EXT4_IOC_RESIZE_FS.

This reverts commit e9f9f61d.