ceph: fix three bugs, two in ceph_vxattrcb_file_layout()

In ceph_vxattrcb_file_layout(), there is a check to determine
whether a preferred PG should be formatted into the output buffer.
That check assumes that a preferred PG number of 0 indicates "no
preference," but that is wrong.  No preference is indicated by a
negative (specifically, -1) PG number.

In addition, if that condition yields true, the preferred value
is formatted into a sized buffer, but the size consumed by the
earlier snprintf() call is not accounted for, opening up the
possibilty of a buffer overrun.

Finally, in ceph_vxattrcb_dir_rctime() where the nanoseconds part of
the time displayed did not include leading 0's, which led to
erroneous (sub-second portion of) time values being shown.

This fixes these three issues:
    http://tracker.newdream.net/issues/2155
    http://tracker.newdream.net/issues/2156
    http://tracker.newdream.net/issues/2157

Signed-off-by: Alex Elder <elder@dreamhost.com>
Reviewed-by: Sage Weil <sage@newdream.net>

fs/ceph/xattr.c

@@ -79,7 +79,7 @@ static size_t ceph_vxattrcb_dir_rbytes(struct ceph_inode_info *ci, char *val,
 static size_t ceph_vxattrcb_dir_rctime(struct ceph_inode_info *ci, char *val,
 				       size_t size)
 {
-	return snprintf(val, size, "%ld.%ld", (long)ci->i_rctime.tv_sec,
+	return snprintf(val, size, "%ld.09%ld", (long)ci->i_rctime.tv_sec,
 			(long)ci->i_rctime.tv_nsec);
 }
 
@@ -118,10 +118,15 @@ static size_t ceph_vxattrcb_file_layout(struct ceph_inode_info *ci, char *val,
 		(unsigned long long)ceph_file_layout_su(ci->i_layout),
 		(unsigned long long)ceph_file_layout_stripe_count(ci->i_layout),
 		(unsigned long long)ceph_file_layout_object_size(ci->i_layout));
-	if (ceph_file_layout_pg_preferred(ci->i_layout))
-		ret += snprintf(val + ret, size, "preferred_osd=%lld\n",
+
+	if (ceph_file_layout_pg_preferred(ci->i_layout) >= 0) {
+		val += ret;
+		size -= ret;
+		ret += snprintf(val, size, "preferred_osd=%lld\n",
 			    (unsigned long long)ceph_file_layout_pg_preferred(
 				    ci->i_layout));
+	}
+
 	return ret;
 }