tmpfs: fix mounts when size is less than the page size

When tmpfs is mounted with a size less than one page, the number of blocks
is set to 0 which makes the tmpfs mount unlimited.  This can lead to a
quick and surprising death if someone typos a tmpfs mount command and
writes too much.

tmpfs can still be mounted as unlimited if size or nr_blocks is exactly 0,
as Documentation/filesystems/tmpfs.txt says.

Hugh: do this by rounding size up instead of down in all cases: which
slightly expands other odd-sized tmpfs mounts, but in a consistent way.

Signed-off-by: Michael Marineau <mike@marineau.org>
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/shmem.c

@@ -2012,7 +2012,7 @@ static int shmem_parse_options(char *options, int *mode, uid_t *uid,
 			}
 			if (*rest)
 				goto bad_val;
-			*blocks = size >> PAGE_CACHE_SHIFT;
+			*blocks = DIV_ROUND_UP(size, PAGE_CACHE_SIZE);
 		} else if (!strcmp(this_char,"nr_blocks")) {
 			*blocks = memparse(value,&rest);
 			if (*rest)