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>
diff --git a/fs/nfs/nfs4file.c b/fs/nfs/nfs4file.c
index a339707..fdfc774 100644
--- a/fs/nfs/nfs4file.c
+++ b/fs/nfs/nfs4file.c
@@ -211,7 +211,7 @@ static loff_t nfs4_file_llseek(struct file *filep, loff_t offset, int whence)
 		ret = nfs42_proc_llseek(filep, offset, whence);
 		if (ret != -ENOTSUPP)
 			return ret;
-		/* Fall through */
+		fallthrough;
 	default:
 		return nfs_file_llseek(filep, offset, whence);
 	}