cifs: set a minimum of 120s for next dns resolution

With commit 506c1da4 ("cifs: use the expiry output of dns_query to
schedule next resolution") and after triggering the first reconnect,
the next async dns resolution of tcp server's hostname would be
scheduled based on dns_resolver's key expiry default, which happens to
default to 5s on most systems that use key.dns_resolver for upcall.

As per key.dns_resolver.conf(5):

       default_ttl=<number>
              The  number  of  seconds  to  set  as the expiration on a cached
              record.  This will be overridden if the program manages  to  re-
              trieve  TTL  information along with the addresses (if, for exam-
              ple, it accesses the DNS directly).  The default is  5  seconds.
              The value must be in the range 1 to INT_MAX.

Make the next async dns resolution no shorter than 120s as we do not
want to be upcalling too often.

Cc: stable@vger.kernel.org
Fixes: 506c1da4

 ("cifs: use the expiry output of dns_query to schedule next resolution")
Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>