Drop an unnecessary mut modifier
Drop an unnecessary mut modifier

Since the result type is not mutable, the pattern does not have to
specify mut.

Signed-off-by: mulhern <7b51bcf507bcd7afb72bf8663752c0ddbeb517f6@redhat.com>

stratis-storage/stratisd,stratis-storage/stratisd,stratis-storage/stratisd