tracefs: Fix potential null dereference in default_file_open()

tracefs: Fix potential null dereference in default_file_open()

The "open" operation in struct file_operations is optional, and
ftrace_event_id_fops does not set it.  In default_file_open(), after
all other checks have passed, return 0 if the underlying struct
file_operations does not implement open.

Fixes: 757ff7244358 ("tracefs: Restrict tracefs when the kernel is …")
References: https://bugs.debian.org/934304
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Gbp-Pq: Topic features/all/lockdown
Gbp-Pq: Name tracefs-fix-potential-null-dereference-in-default_fi.patch