client: set client_try_dentry_invalidate to false by default

client: set client_try_dentry_invalidate to false by default

By default, ceph-fuse uses side effect of 'dentry invalidation' to
trim kernel dcache if it runs on kernel < 3.18. The implemention of
kernel function d_invalidate() changed in 3.18 kernel, the method no
longer works for upstream kernel >= 3.18.

RHEL 3.10 kernel includes backport of patches that change implemention
of d_invalidate(). So checking kernel version to decide if 'dentry
invalidation' method works is unreliable.

Fixes: http://tracker.ceph.com/issues/21423
Signed-off-by: "Yan, Zheng" <zyan@redhat.com>