fs/proc/internal.h: fix up comment

Document what ->pde_unload_lock actually does.

Link: http://lkml.kernel.org/r/20180103185120.GB31849@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>