libext2fs: fix alloc_allocate_group_table() if the flexbg_offset wraps

libext2fs: fix alloc_allocate_group_table() if the flexbg_offset wraps

If the previous block group's inode table ends at the very end of file
system, wrap around to the beginning of the flex_bg.

This fixes a bug was tickled by:

mke2fs.conf:
	frontload = {
		features = extent,huge_file,flex_bg,uninit_bg,dir_nlink,extra_isize,^resize_inode,sparse_super2
		hash_alg = half_md4
		num_backup_sb = 0
		packed_meta_blocks = 1
		inode_ratio = 4194304
		flex_bg_size = 262144
	}

mke2fs -T frontload /tmp/foo.img 2T
resize2fs -M /tmp/foo.img
resize2fs -M /tmp/foo.img

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