ring-buffer: check for less than two in size allocation

The ring buffer must have at least two pages allocated for the
reader page swap to work.

The page count check will miss the case of a zero size passed in.
Even though a zero size ring buffer would probably fail an allocation,
making the min size check for less than two instead of equal to one makes
the code a bit more robust.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>