uml: fix initrd printk

If the initrd file has zero-length, the error message should contain
the filepath.

Cc: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Johann Felix Soden <johfel@users.sourceforge.net>
Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>