VM: add "vm_brk()" helper function

It does the same thing as "do_brk()", except it handles the VM locking
too.

It turns out that all external callers want that anyway, so we can make
do_brk() static to just mm/mmap.c while at it.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mm.h

@@ -1415,7 +1415,8 @@ out:
 
 extern int do_munmap(struct mm_struct *, unsigned long, size_t);
 
-extern unsigned long do_brk(unsigned long, unsigned long);
+/* These take the mm semaphore themselves */
+extern unsigned long vm_brk(unsigned long, unsigned long);
 
 /* truncate.c */
 extern void truncate_inode_pages(struct address_space *, loff_t);