vfs: split do_lookup()

Split do_lookup() into two functions:

  lookup_fast() - does cached lookup without i_mutex
  lookup_slow() - does lookup with i_mutex

Both follow managed dentries.

The new functions are needed by atomic_open.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>