ext3: Cleanup ext3_setup_super()

Fix mount-count check to emit warning only if s_max_mnt_count
is greater than 0 according to man tune2fs(8). Also removes
unnecessary casts.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>