treewide: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1]. Also, remove unnecessary
fall-through markings when it is the case.

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

fs/nfs_common/nfsacl.c

@@ -237,7 +237,7 @@ posix_acl_from_nfsacl(struct posix_acl *acl)
 				break;
 			case ACL_MASK:
 				mask = pa;
-				/* fall through */
+				fallthrough;
 			case ACL_OTHER:
 				break;
 		}