Add a variable, ceph_nfs_disable_caching, that if set to true
disables ganesha's directory and attribute caching as much as
possible.
Also, disable caching done by ganesha, when 'nfs_file_gw'
variable is true, i.e., when Ganesha is used as CephFS's gateway.
This is the recommended Ganesha setting as libcephfs already caches
information. And doing so helps avoid cache incoherency issues
especially with clustered ganesha over CephFS.
Fixes: https://tracker.ceph.com/issues/23393 Signed-off-by: Ramana Raja <rraja@redhat.com>