LU-9859 libcfs: don't call unshare_fs_struct()

LU-9859 libcfs: don't call unshare_fs_struct()

A kthread runs with the same fs_struct as init.
It is only helpful to unshare this if the thread
will change one of the fields in the fs_struct:
 root directory
 current working directory
 umask.

No lustre kthread changes any of these, so there is
no need to call unshare_fs_struct().

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I7309b6ed184b14a272bad7dc5149ad36281f948e
Reviewed-on: https://review.whamcloud.com/39132
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>