Add MS_MANDLOCK mount failure message

Commit torvalds/linux@9e8925b6 allowed for kernels to be built
without support for mandatory locking (MS_MANDLOCK).  This will
result in 'zfs mount' failing when the nbmand=on property is set
and the kernel is build without CONFIG_MANDATORY_FILE_LOCKING.

Unfortunately we can't detect prior to the mount(2) system call
if the kernel was built with this support.  The best we can do
is check if the mount failed with EPERM and if we passed 'mand'
as a mount option and print a more useful error message.  e.g.

  filesystem 'tank' has the 'nbmand=on' property set, this mount
  option may be disabled in your kernel.  Use 'zfs set nbmand=off'
  to disable this option and try to mount the filesystem again.

Additionally, switch the default error message case to use
strerror() to produce a more human readable message.

Signed-off-by: Brian Behlendorf <[email protected]>
Issue openzfs#4729

cmd/mount_zfs/mount_zfs.c

@@ -607,10 +607,23 @@ main(int argc, char **argv)
 				    "failed for unknown reason.\n"), dataset);
 			}
 			return (MOUNT_SYSERR);
+#ifdef MS_MANDLOCK
+		case EPERM:
+			if (mntflags & MS_MANDLOCK) {
+				(void) fprintf(stderr, gettext("filesystem "
+				    "'%s' has the 'nbmand=on' property set, "
+				    "this mount\noption may be disabled in "
+				    "your kernel.  Use 'zfs set nbmand=off'\n"
+				    "to disable this option and try to "
+				    "mount the filesystem again.\n"), dataset);
+				return (MOUNT_SYSERR);
+			}
+			/* fallthru */
+#endif
 		default:
 			(void) fprintf(stderr, gettext("filesystem "
-			    "'%s' can not be mounted due to error "
-			    "%d\n"), dataset, errno);
+			    "'%s' can not be mounted: %s\n"), dataset,
+			    strerror(errno));
 			return (MOUNT_USAGE);
 		}
 	}