CRED: Use creds in file structs

Attach creds to file structs and discard f_uid/f_gid.

file_operations::open() methods (such as hppfs_open()) should use file->f_cred
rather than current_cred().  At the moment file->f_cred will be current_cred()
at this point.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: James Morris <jmorris@namei.org>
Signed-off-by: James Morris <jmorris@namei.org>