ext4: avoid race conditions when remounting with options that change dax

Trying to change dax mount options when remounting could allow mount
options to be enabled for a small amount of time, and then the mount
option change would be reverted.

In the case of "mount -o remount,dax", this can cause a race where
files would temporarily treated as DAX --- and then not.

Cc: stable@kernel.org
Reported-by:  <syzbot+bca9799bf129256190da@syzkaller.appspotmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>