implicit conversion from 'boolean_t' to 'ds_hold_flags_t'

Build error on illumos with gcc 10 did reveal:

In function 'dmu_objset_refresh_ownership':
../../common/fs/zfs/dmu_objset.c:857:25: error: implicit conversion
from 'boolean_t' to 'ds_hold_flags_t' {aka 'enum ds_hold_flags'}
[-Werror=enum-conversion]
      857 |  dsl_dataset_disown(ds, decrypt, tag);
          |                         ^~~~~~~
cc1: all warnings being treated as errors

libzfs_input_check.c: In function 'zfs_ioc_input_tests':
libzfs_input_check.c:754:28: error: implicit conversion from
'enum dmu_objset_type' to 'enum lzc_dataset_type'
[-Werror=enum-conversion]
  754 |  err = lzc_create(dataset, DMU_OST_ZFS, NULL, NULL, 0);
      |                            ^~~~~~~~~~~
cc1: all warnings being treated as errors

The same issue is present in openzfs, and also the same issue about
ds_hold_flags_t, which currently defines exactly one valid value.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Toomas Soome <tsoome@me.com>
Closes #11406

include/sys/dsl_dataset.h

@@ -316,6 +316,7 @@ typedef struct dsl_dataset_snapshot_arg {
 
 /* flags for holding the dataset */
 typedef enum ds_hold_flags {
+	DS_HOLD_FLAG_NONE	= 0 << 0,
 	DS_HOLD_FLAG_DECRYPT	= 1 << 0 /* needs access to encrypted data */
 } ds_hold_flags_t;