switch unlock_mount() to namespace_unlock(), convert all umount_tree() callers

which allows to kill the last argument of umount_tree() and make release_mounts()
static.

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