rename user_path_umountat() to user_path_mountpoint_at()

... and move the extern from linux/namei.h to fs/internal.h,
along with that of vfs_path_lookup().

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

diff --cc fs/internal.h
Simple merge

diff --cc fs/namei.c
Simple merge

diff --cc fs/namespace.c
Simple merge

diff --cc include/linux/namei.h
Simple merge