mount_mfs.8: remove self-reference
Change .Xr instances to .Nm, which quiets several mandoc warnings.
Reviewed by: mhorne
MFC after: 3 days
Pull Request: https://github.com/freebsd/freebsd-src/pull/1135
(cherry picked from commit 2ebc7806a1e4f4cd82a21a123ee5078ca6c4b113)