cifs: reorganize get_cifs_acl

Thus spake Christoph:

"But this whole set_cifs_acl function is a real mess anyway and needs
some splitting up."

With this change too, it's possible to call acl_to_uid_mode() with a
NULL inode pointer. That (or something close to it) will eventually be
necessary when cifs_get_inode_info is reorganized.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Shirish Pargaonkar <shirishp@us.ibm.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>