vfs: spread struct mount - get_dominating_id / do_make_slave

next pile of horrors, similar to mnt_parent one; this time it's
mnt_master.

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

diff --cc fs/namespace.c
Simple merge

diff --cc fs/pnode.c
Simple merge

diff --cc fs/pnode.h
Simple merge