propagate_mnt(): get rid of globals

TBH, I'm not sure it makes the things better, but...

Fixed a stale comment, while we are at it - propagate_mnt() does *not*
use ->mnt_list for linkage and it does not attach the secondaries to
the original - they go into the caller-supplied list.  It had gone
stale before the entire thing got merged into the tree...

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