UBIFS: fix output format of INUM_WATERMARK

The INUM_WATERMARK is a unsigned 32bit value, `%d' prints it as negatave:
[  103.682255] UBIFS warning (ubi0:0 pid 691): ubifs_new_inode: running out of inode numbers (current 122763, max -256)

Fix it as:
[  154.422940] UBIFS warning (ubi0:0 pid 688): ubifs_new_inode: running out of inode numbers (current 122765, max 4294967040)

Signed-off-by: Sheng Yong <shengyong1@huawei.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>