cifs: set MAY_SIGN when sec=krb5

cifs: set MAY_SIGN when sec=krb5

commit 0b7bc84000d71f3647ca33ab1bf5bd928535c846 upstream.

Setting this secFlg allows usage of dfs where some servers require
signing and others don't.

Signed-off-by: Martijn de Gouw <martijn.de.gouw@prodrive.nl>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>
[Joseph Salisbury: This backport was done so including mainline commit
8830d7e07a5e38bc47650a7554b7c1cfd49902bf is not needed.]
BugLink: http://bugs.launchpad.net/bugs/1285723
Signed-off-by: Joseph Salisbury <joseph.salisbury@canonical.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>