dm: Convert to use ERR_CAST()

Use ERR_CAST() as it is designed for casting an error pointer to
another type.

This macro utilizes the __force and __must_check modifiers, which instruct
the compiler to verify for errors at the locations where it is employed.

Signed-off-by: Yuesong Li <liyuesong@vivo.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>