eventfs: Implement tracefs_inode_cache

Create a kmem cache of tracefs_inodes. To be more efficient, as there are
lots of tracefs inodes, create its own cache. This also allows to see how
many tracefs inodes have been created.

Add helper functions:
tracefs_alloc_inode()
tracefs_free_inode()
get_tracefs()

Link: https://lkml.kernel.org/r/1690568452-46553-3-git-send-email-akaher@vmware.com

Signed-off-by: Ajay Kaher <akaher@vmware.com>
Co-developed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Tested-by: Ching-lin Yu <chinglinyu@google.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>