Anonymous mappings should ignore offset but shared anonymous mapping
forgot to clear it and makes the following legit test program trigger
SIGBUS.

 #include <sys/mman.h>
 #include <stdio.h>
 #include <errno.h>

 #define PAGE_SIZE	4096

 int main(void)
 {
	 char *p;
	 int i;

	 p = mmap(NULL, 2 * PAGE_SIZE, PROT_READ|PROT_WRITE,
		  MAP_SHARED|MAP_ANONYMOUS, -1, PAGE_SIZE);
	 if (p == MAP_FAILED) {
		 perror("mmap");
		 return 1;
	 }

	 for (i = 0; i < 2; i++) {
		 printf("page %d\n", i);
		 p[i * 4096] = i;
	 }
	 return 0;
 }

Fix it.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Hugh Dickins <hugh@veritas.com>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>