mm: Fix problem of parameter in note

'current' is a pointer, so the right form is  'down_write(&current->mm->mmap_sem)'.

Signed-off-by: Jianjun Kong <jianjun@zeuux.org>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>