Handle ECKSUM as new EZFS_CKSUM ‒ "insufficient replicas"

Add a meaningful error message for ECKSUM to common error messages.

Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #6805 
Closes #13808
Closes #13898

include/libzfs.h

@@ -151,6 +151,7 @@ typedef enum zfs_error {
 	EZFS_REBUILDING,	/* resilvering (sequential reconstrution) */
 	EZFS_VDEV_NOTSUP,	/* ops not supported for this type of vdev */
 	EZFS_NOT_USER_NAMESPACE,	/* a file is not a user namespace */
+	EZFS_CKSUM,		/* insufficient replicas */
 	EZFS_UNKNOWN
 } zfs_error_t;