ptp: prevent possible ABBA deadlock in ptp_clock_freerun()

syzbot reported the following ABBA deadlock:

CPU0 CPU1
---- ----
n_vclocks_store()
lock(&ptp->n_vclocks_mux) [1]
(physical clock)
pc_clock_adjtime()
lock(&clk->rwsem) [2]
(physical clock)
...
ptp_clock_freerun()
ptp_vclock_in_use()
lock(&ptp->n_vclocks_mux) [3]
(physical clock)
ptp_clock_unregister()
posix_clock_unregister()
lock(&clk->rwsem) [4]
(virtual clock)

Since ptp virtual clock is registered only under ptp physical clock, both
ptp_clock and posix_clock must be physical clocks for ptp_vclock_in_use()
to lock &ptp->n_vclocks_mux and check ptp->n_vclocks.

However, when unregistering vclocks in n_vclocks_store(), the locking
ptp->n_vclocks_mux is a physical clock lock, but clk->rwsem of
ptp_clock_unregister() called through device_for_each_child_reverse()
is a virtual clock lock.

Therefore, clk->rwsem used in CPU0 and clk->rwsem used in CPU1 are
different locks, but in lockdep, a false positive occurs because the
possibility of deadlock is determined through lock-class.

To solve this, lock subclass annotation must be added to the posix_clock
rwsem of the vclock.

Reported-by: syzbot+7cfb66a237c4a5fb22ad@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=7cfb66a237c4a5fb22ad
Fixes: 73f37068d540 ("ptp: support ptp physical/virtual clocks conversion")
Signed-off-by: Jeongjun Park <aha310510@gmail.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20250728062649.469882-1-aha310510@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>