namei: add audit_inode to all branches in path_lookup

Main change is in path_lookup: added a goto to do audit_inode
instead of return statement, when emul_lookup_dentry for root
is successful.The existing code does audit_inode only when
lookup is done in normal root or cwd.

Other changes: Some lookup routines are returning zero on success,
and some are returning zero on failure. I documented the related
function signatures in this code path, so that one can glance over
abstract functions without understanding the entire code.

Signed-off-by: Prasanna Meda <pmeda@akamai.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>