src/t_mmap_dio: fix incorrect argument count check

src/t_mmap_dio: fix incorrect argument count check

t_mmap_dio.c actually requires 4 arguments, not 3 as the current
check enforces:

usage: t_mmap_dio <src file> <dest file> <size> <msg>
open src(No such file or directory) len 0 (null)

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Fixes: 456581661b4d ("xfs: test per-inode DAX flag by IO")
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>