Add a missing EXPORT_SYMBOL.

I must be the first person that wants to use this function :-)
Signed-off-by: NHans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>