ring_buffer: fix ring_buffer_event_length()

Function ring_buffer_event_length() provides an interface to detect
the length of data stored in an entry. However, the length contains
offsets depending on the internal usage. This makes it unusable. This
patch fixes this and now ring_buffer_event_length() returns the
alligned length that has been used in ring_buffer_lock_reserve().

Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Robert Richter <robert.richter@amd.com>