cifs: switch build_path_from_dentry() to using dentry_path_raw()

The cost is that we might need to flip '/' to '\\' in more than
just the prefix.  Needs profiling, but I suspect that we won't
get slowdown on that.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Steve French <stfrench@microsoft.com>