Rename fallthrough to zfs_fallthrough

Unfortunately macOS has obj-C keyword "fallthrough" in the OS headers.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Damian Szuberski <szuberskidamian@gmail.com>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Closes #13097

module/os/linux/zfs/zfs_acl.c

@@ -269,7 +269,7 @@ zfs_ace_fuid_size(void *acep)
 		    entry_type == OWNING_GROUP ||
 		    entry_type == ACE_EVERYONE)
 			return (sizeof (zfs_ace_hdr_t));
-		fallthrough;
+		zfs_fallthrough;
 	default:
 		return (sizeof (zfs_ace_t));
 	}
@@ -2317,7 +2317,7 @@ zfs_zaccess_aces_check(znode_t *zp, uint32_t *working_mode,
 			break;
 		case OWNING_GROUP:
 			who = gowner;
-			fallthrough;
+			zfs_fallthrough;
 		case ACE_IDENTIFIER_GROUP:
 			checkit = zfs_groupmember(zfsvfs, who, cr);
 			break;

module/os/linux/zfs/zfs_znode.c

@@ -430,7 +430,7 @@ zfs_inode_set_ops(zfsvfs_t *zfsvfs, struct inode *ip)
 	case S_IFBLK:
 		(void) sa_lookup(ITOZ(ip)->z_sa_hdl, SA_ZPL_RDEV(zfsvfs), &rdev,
 		    sizeof (rdev));
-		fallthrough;
+		zfs_fallthrough;
 	case S_IFIFO:
 	case S_IFSOCK:
 		init_special_inode(ip, ip->i_mode, rdev);