smb3: allow controlling length of time directory entries are cached with dir leases

Currently with directory leases we cache directory contents for a fixed period
of time (default 30 seconds) but for many workloads this is too short.  Allow
configuring the maximum amount of time directory entries are cached when a
directory lease is held on that directory. Add module load parm "max_dir_cache"

For example to set the timeout to 10 minutes you would do:

  echo 600 > /sys/module/cifs/parameters/dir_cache_timeout

or to disable caching directory contents:

  echo 0 > /sys/module/cifs/parameters/dir_cache_timeout

Reviewed-by: Bharath SM <bharathsm@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>