resize2fs: take s_first_data_block into account for resize2fs -M

resize2fs: take s_first_data_block into account for resize2fs -M

calculate_minimum_resize_size() forgot to account s_first_data_block
into minimum filesystem size. Thus in case the size of filesystem was
such that the last group had the minimal size (50 blocks + metadata
overhead), the code in adjust_fs_info() decided the group is unneeded,
removed it, and in some cases the resizing then failed with ENOSPC.

Fix the issue by properly accounting for s_first_data_block in
calculate_minimum_resize_size().

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>