securityfs: fix use-after-free on symlink traversal

securityfs: fix use-after-free on symlink traversal

BugLink: https://bugs.launchpad.net/bugs/1838576
[ Upstream commit 46c874419652bbefdfed17420fd6e88d8a31d9ec ]

symlink body shouldn't be freed without an RCU delay.  Switch securityfs
to ->destroy_inode() and use of call_rcu(); free both the inode and symlink
body in the callback.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>