watch_queue: Actually free the watch

In the Linux kernel, the following vulnerability has been resolved:

watch_queue: Actually free the watch

free_watch() does everything barring actually freeing the watch object. Fix

this by adding the missing kfree.

kmemleak produces a report something like the following. Note that as an

address can be seen in the first word, the watch would appear to have gone

through call_rcu().

BUG: memory leak

unreferenced object 0xffff88810ce4a200 (size 96):

comm "syz-executor352", pid 3605, jiffies 4294947473 (age 13.720s)

hex dump (first 32 bytes):

e0 82 48 0d 81 88 ff ff 00 00 00 00 00 00 00 00 ..H.............

80 a2 e4 0c 81 88 ff ff 00 00 00 00 00 00 00 00 ................

backtrace:

[<ffffffff8214e6cc>] kmalloc include/linux/slab.h:581 [inline]

[<ffffffff8214e6cc>] kzalloc include/linux/slab.h:714 [inline]

[<ffffffff8214e6cc>] keyctl_watch_key+0xec/0x2e0 security/keys/keyctl.c:1800

[<ffffffff8214ec84>] __do_sys_keyctl+0x3c4/0x490 security/keys/keyctl.c:2016

[<ffffffff84493a25>] do_syscall_x64 arch/x86/entry/common.c:50 [inline]

[<ffffffff84493a25>] do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80

[<ffffffff84600068>] entry_SYSCALL_64_after_hwframe+0x44/0xae