tracing: Move mutex to protect against resetting of seq data

commit 1245800c upstream.

The iter->seq can be reset outside the protection of the mutex. So can
reading of user data. Move the mutex up to the beginning of the function.

Fixes: d7350c3f ("tracing/core: make the read callbacks reentrants")
Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>