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

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

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.