fs: expose LOOKUP_CACHED through openat2() RESOLVE_CACHED

fs: expose LOOKUP_CACHED through openat2() RESOLVE_CACHED

Now that we support non-blocking path resolution internally, expose it
via openat2() in the struct open_how ->resolve flags. This allows
applications using openat2() to limit path resolution to the extent that
it is already cached.

If the lookup cannot be satisfied in a non-blocking manner, openat2(2)
will return -1/-EAGAIN.

Cc: Al Viro <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>
Signed-off-by: Al Viro <[email protected]>