FS: lookup_mnt() is only used in the core fs routines now

lookup_mnt() is only used in the core fs routines now, so it doesn't need to
be globally declared anymore.  It isn't exported to modules at the moment, so
nothing that can be modularised seems to be using it.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>