fuse: make DAX mount option a tri-state

We add 'always', 'never', and 'inode' (default). '-o dax' continues to
operate the same which is equivalent to 'always'.

The following behavior is consistent with that on ext4/xfs:

 - The default behavior (when neither '-o dax' nor
   '-o dax=always|never|inode' option is specified) is equal to 'inode'
   mode, while 'dax=inode' won't be printed among the mount option list.

 - The 'inode' mode is only advisory. It will silently fallback to 'never'
   mode if fuse server doesn't support that.

Also noted that by the time of this commit, 'inode' mode is actually equal
to 'always' mode, before the per inode DAX flag is introduced in the
following patch.

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>