Fix optional "force" arg handing in zfs_ioc_pool_sync()

[behlendorf]

Motivation and Context

Issue #11281

Description

When checking if the "force" argument is set it is sufficient
to use nvlist_exists(). fnvlist_lookup_boolean_value() should
not be used in this case since it's never allowed to fail which
can happen when the provided nvlist does not set NV_UNIQUE_NAME.
This is consistent with the boolean option handling for the
other ioctls.

How Has This Been Tested?

Using the test case provided in #11281.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[ghost]

nvpair_type DATA_TYPE_BOOLEAN != DATA_TYPE_BOOLEAN_VALUE, you have to check its value because it can be false.

zfs/lib/libzpool/util.c

Lines 222 to 240 in dcbf847

	/*
	* Use ZFS_IOC_POOL_SYNC to confirm if a pool is active
	*/
	fd = open(ZFS_DEV, O_RDWR);
	if (fd < 0)
	return (-1);
	zcp = umem_zalloc(sizeof (zfs_cmd_t), UMEM_NOFAIL);
	innvl = fnvlist_alloc();
	fnvlist_add_boolean_value(innvl, "force", B_FALSE);
	(void) strlcpy(zcp->zc_name, name, sizeof (zcp->zc_name));
	packed = fnvlist_pack(innvl, &size);
	zcp->zc_nvlist_src = (uint64_t)(uintptr_t)packed;
	zcp->zc_nvlist_src_size = size;
	ret = zfs_ioctl_fd(fd, ZFS_IOC_POOL_SYNC, zcp);

[behlendorf]

It does appear we set it this way. Updated.