cachefiles: Fix oops in trace_cachefiles_mark_buried due to NULL object

In cachefiles_mark_object_buried, the dentry in question may not have an
owner, and thus our cachefiles_object pointer may be NULL when calling
the tracepoint, in which case we will also not have a valid debug_id to
print in the tracepoint.

Check for NULL object in the tracepoint and if so, just set debug_id to
MAX_UINT as was done in 2908f5e101e3 ("fscache: Add a cookie debug ID
and use that in traces").

This fixes the following oops:

FS-Cache: Cache "mycache" added (type cachefiles)
CacheFiles: File cache on vdc registered
...
Workqueue: fscache_object fscache_object_work_func [fscache]
RIP: 0010:trace_event_raw_event_cachefiles_mark_buried+0x4e/0xa0 [cachefiles]
....
Call Trace:
cachefiles_mark_object_buried+0xa5/0xb0 [cachefiles]
cachefiles_bury_object+0x270/0x430 [cachefiles]
cachefiles_walk_to_object+0x195/0x9c0 [cachefiles]
cachefiles_lookup_object+0x5a/0xc0 [cachefiles]
fscache_look_up_object+0xd7/0x160 [fscache]
fscache_object_work_func+0xb2/0x340 [fscache]
process_one_work+0x1f1/0x390
worker_thread+0x53/0x3e0
kthread+0x127/0x150

Fixes: 2908f5e101e3 ("fscache: Add a cookie debug ID and use that in traces")
Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-cachefs@redhat.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>