split __lookup_mnt() in two functions

Instead of passing the direction as argument (and checking it on every
step through the hash chain), just have separate __lookup_mnt() and
__lookup_mnt_last().  And use the standard iterators...

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

diff --cc fs/mount.h
Simple merge

diff --cc fs/namei.c
Simple merge

diff --cc fs/namespace.c
Simple merge

diff --cc fs/pnode.c
Simple merge