fuse: move option checking into fuse_fill_super()

Checking whether the "fd=", "rootmode=", "user_id=" and "group_id=" mount
options are present can be moved from fuse_get_tree() into
fuse_fill_super() where the value of the options are consumed.

This relaxes semantics of reusing a fuse blockdev mount using the device
name.  Before this patch presence of these options were enforced but values
ignored, after this patch these options are completely ignored in this
case.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>