mm: use dedicated helper to access rlimit value

Use rlimit() helper instead of manually writing whole chain from current
task to rlim_cur.

Link: http://lkml.kernel.org/r/20170705172811.8027-1-k.opasiak@samsung.com
Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -2177,7 +2177,6 @@ static int acct_stack_growth(struct vm_area_struct *vma,
 			     unsigned long size, unsigned long grow)
 {
 	struct mm_struct *mm = vma->vm_mm;
-	struct rlimit *rlim = current->signal->rlim;
 	unsigned long new_start;
 
 	/* address space limit tests */
@@ -2185,7 +2184,7 @@ static int acct_stack_growth(struct vm_area_struct *vma,
 		return -ENOMEM;
 
 	/* Stack limit test */
-	if (size > READ_ONCE(rlim[RLIMIT_STACK].rlim_cur))
+	if (size > rlimit(RLIMIT_STACK))
 		return -ENOMEM;
 
 	/* mlock limit tests */
@@ -2193,7 +2192,7 @@ static int acct_stack_growth(struct vm_area_struct *vma,
 		unsigned long locked;
 		unsigned long limit;
 		locked = mm->locked_vm + grow;
-		limit = READ_ONCE(rlim[RLIMIT_MEMLOCK].rlim_cur);
+		limit = rlimit(RLIMIT_MEMLOCK);
 		limit >>= PAGE_SHIFT;
 		if (locked > limit && !capable(CAP_IPC_LOCK))
 			return -ENOMEM;