eventfs: Fix a possible null pointer dereference in eventfs_find_events()

In function eventfs_find_events,there is a potential null pointer
that may be caused by calling update_events_attr which will perform
some operations on the members of the ei struct when ei is NULL.

Hence,When ei->is_freed is set,return NULL directly.

Link: https://lore.kernel.org/linux-trace-kernel/20240513053338.63017-1-hao.ge@linux.dev

Cc: stable@vger.kernel.org
Fixes: 8186fff7

 ("tracefs/eventfs: Use root and instance inodes as default ownership")
Signed-off-by: Hao Ge <gehao@kylinos.cn>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>