Use fallthrough macro

As of the Linux 5.9 kernel a fallthrough macro has been added which
should be used to anotate all intentional fallthrough paths.  Once
all of the kernel code paths have been updated to use fallthrough
the -Wimplicit-fallthrough option will because the default.  To
avoid warnings in the OpenZFS code base when this happens apply
the fallthrough macro.

Additional reading: https://lwn.net/Articles/794944/

Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12441

module/os/freebsd/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*/
+		fallthrough;
 	default:
 		return (sizeof (zfs_ace_t));
 	}
@@ -2153,7 +2153,7 @@ zfs_zaccess_aces_check(znode_t *zp, uint32_t *working_mode,
 			break;
 		case OWNING_GROUP:
 			who = gowner;
-			/*FALLTHROUGH*/
+			fallthrough;
 		case ACE_IDENTIFIER_GROUP:
 			checkit = zfs_groupmember(zfsvfs, who, cr);
 			break;

module/os/freebsd/zfs/zfs_vnops_os.c

@@ -974,7 +974,7 @@ zfs_lookup(vnode_t *dvp, const char *nm, vnode_t **vpp,
 				cnp->cn_flags |= SAVENAME;
 				break;
 			}
-			/* FALLTHROUGH */
+			fallthrough;
 		case DELETE:
 			if (error == 0)
 				cnp->cn_flags |= SAVENAME;