libfsimage: fix clang 10 build

libfsimage: fix clang 10 build

clang complains with:

fsys_zfs.c:826:2: error: converting the enum constant to a boolean [-Werror,-Wint-in-bool-context]
        VERIFY_DN_TYPE(dn, DMU_OT_PLAIN_FILE_CONTENTS);
        ^
/wrkdirs/usr/ports/sysutils/xen-tools/work/xen-4.13.0/tools/libfsimage/zfs/../../../tools/libfsimage/zfs/fsys_zfs.h:74:11: note: expanded from macro 'VERIFY_DN_TYPE'
        if (type && (dnp)->dn_type != type) { \
                 ^
1 error generated.

Fix this by not forcing an implicit conversion of the enum into a
boolean and instead comparing with the 0 enumerator.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wl@xen.org>