Add feature check for 'zpool resilver' command

The 'zpool resilver' command requires that the resilver_defer
feature is active on the pool. Unfortunately, the check for
this was left out of the original patch. This commit simply
corrects this so that the command properly returns an error
in this case.

Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #8700

include/libzfs.h

@@ -146,6 +146,7 @@ typedef enum zfs_error {
 	EZFS_TRIMMING,		/* currently trimming */
 	EZFS_NO_TRIM,		/* no active trim */
 	EZFS_TRIM_NOTSUP,	/* device does not support trim */
+	EZFS_NO_RESILVER_DEFER,	/* pool doesn't support resilver_defer */
 	EZFS_UNKNOWN
 } zfs_error_t;