kill kernfs_pin_sb()

mainline inclusion
from mainline-5.1-rc1
commit 6d7fbce7
category: bugfix
bugzilla: 30217
CVE: NA
---------------------------

unused now and impossible to use safely anyway.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: yangerkun <yangerkun@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>