cifs: get rid of unneeded conditional in cifs_get_num_sgs()

Just have @skip set to 0 after first iterations of the two nested
loops.

Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>