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

@@ -150,6 +150,7 @@ typedef enum zfs_error {
 	EZFS_NO_RESILVER_DEFER,	/* pool doesn't support resilver_defer */
 	EZFS_EXPORT_IN_PROGRESS,	/* currently exporting the pool */
 	EZFS_REBUILDING,	/* resilvering (sequential reconstrution) */
+	EZFS_CKSUM,		/* insufficient replicas */
 	EZFS_UNKNOWN
 } zfs_error_t;