separate -ESTALE/-ECHILD retries in do_filp_open() from real work

new helper: path_openat().  Does what do_filp_open() does, except
that it tries only the walk mode (RCU/normal/force revalidation)
it had been told to.

Both create and non-create branches are using path_lookupat() now.
Fixed the double audit_inode() in non-create branch.

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