cifs: convert cifs_get_inode_info and non-posix readdir to use cifs_iget

Rather than allocating an inode and filling it out, have
cifs_get_inode_info fill out a cifs_fattr and call cifs_iget. This means
a pretty hefty reorganization of cifs_get_inode_info.

For the readdir codepath, add a couple of new functions for filling out
cifs_fattr's from different FindFile response infolevels.

Finally, remove cifs_new_inode since there are no more callers.
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Steve French <sfrench@us.ibm.com>