ext4: don't warn about mnt_count if it has been disabled

Currently, if we mkfs a new ext4 volume with s_max_mnt_count set to
zero, and mount it for the first time, we will get the warning:

	maximal mount count reached, running e2fsck is recommended

It is really misleading. So change the check so that it won't warn in
that case.

Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>