There are a couple of places where JBD has to check to see whether an unneeded
memory allocation was performed.  Usually it _was_ needed, so we end up
calling kfree(NULL).  We can micro-optimise that by checking the pointer
before calling kfree().

Thanks to Steven Rostedt <rostedt@goodmis.org> for identifying this.
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>