propagate_mnt(): get rid of last_dest

Its only use is choosing the type of copy - CL_MAKE_SHARED if there
already is a copy in that peer group, CL_SLAVE or CL_SLAVE | CL_MAKE_SHARED
otherwise.

But that's easy to keep track of - just set type in the beginning of group
and reset to CL_MAKE_SHARED after the first created secondary in it...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>