proc: fix type of struct pde_opener::closing field

struct pde_opener::closing is boolean.

Link: http://lkml.kernel.org/r/20161029155439.GB1246@avx2


Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>