cifs: Make echo interval tunable

Currently the echo interval is set to 60 seconds using a macro. This
setting determines the interval at which echo requests are sent to the
server on an idling connection. This setting also affects the time
required for a connection to an unresponsive server to timeout.

Making this setting a tunable allows users to control the echo interval
times as well as control the time after which the connecting to an
unresponsive server times out.

To set echo interval, pass the echo_interval=n mount option.

Version four of the patch.
v2: Change MIN and MAX timeout values
v3: Remove incorrect comment in cifs_get_tcp_session
v4: Fix bug in setting echo_intervalw

Signed-off-by: Sachin Prabhu <sprabhu@redhat.com>
Acked-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>