ext4: fix online resizing for bigalloc file systems with a 1k block size

An online resize of a file system with the bigalloc feature enabled
and a 1k block size would be refused since ext4_resize_begin() did not
understand s_first_data_block is 0 for all bigalloc file systems, even
when the block size is 1k.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org