libmount: (fuse) follow only user_id= on umount

libmount: (fuse) follow only user_id= on umount

The option user= is already handled by evaluate_permissions() and by
classic mount and umount usermount support. It seems we do not need
to duplicate support for user= in is_fuse_usermount().

The option user_id= is fuse specific and it's maintained by
libfuse/kernel in /proc/self/mountinfo. This is feature we need to
support in umount(8).

Addresses: https://github.com/karelzak/util-linux/pull/705
Signed-off-by: Karel Zak <kzak@redhat.com>