xfs: Fix multiple fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix
the following warnings by replacing /* fallthrough */ comments,
and its variants, with the new pseudo-keyword macro fallthrough:

fs/xfs/libxfs/xfs_attr.c:487:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
fs/xfs/libxfs/xfs_attr.c:500:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
fs/xfs/libxfs/xfs_attr.c:532:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
fs/xfs/libxfs/xfs_attr.c:594:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
fs/xfs/libxfs/xfs_attr.c:607:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
fs/xfs/libxfs/xfs_attr.c:1410:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
fs/xfs/libxfs/xfs_attr.c:1445:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
fs/xfs/libxfs/xfs_attr.c:1473:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]

Notice that Clang doesn't recognize /* fallthrough */ comments as
implicit fall-through markings, so in order to globally enable
-Wimplicit-fallthrough for Clang, these comments need to be
replaced with fallthrough; in the whole codebase.

Link: https://github.com/KSPP/linux/issues/115Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

fs/xfs/libxfs/xfs_attr.c

@@ -483,7 +483,7 @@ xfs_attr_set_iter(
 		if (error)
 			return error;
 
-		/* fallthrough */
+		fallthrough;
 	case XFS_DAS_RM_LBLK:
 		/* Set state in case xfs_attr_rmtval_remove returns -EAGAIN */
 		dac->dela_state = XFS_DAS_RM_LBLK;
@@ -496,7 +496,7 @@ xfs_attr_set_iter(
 			return -EAGAIN;
 		}
 
-		/* fallthrough */
+		fallthrough;
 	case XFS_DAS_RD_LEAF:
 		/*
 		 * This is the last step for leaf format. Read the block with
@@ -528,7 +528,7 @@ xfs_attr_set_iter(
 				return error;
 		}
 
-		/* fallthrough */
+		fallthrough;
 	case XFS_DAS_ALLOC_NODE:
 		/*
 		 * If there was an out-of-line value, allocate the blocks we
@@ -590,7 +590,7 @@ xfs_attr_set_iter(
 		if (error)
 			return error;
 
-		/* fallthrough */
+		fallthrough;
 	case XFS_DAS_RM_NBLK:
 		/* Set state in case xfs_attr_rmtval_remove returns -EAGAIN */
 		dac->dela_state = XFS_DAS_RM_NBLK;
@@ -603,7 +603,7 @@ xfs_attr_set_iter(
 			return -EAGAIN;
 		}
 
-		/* fallthrough */
+		fallthrough;
 	case XFS_DAS_CLR_FLAG:
 		/*
 		 * The last state for node format. Look up the old attr and
@@ -1406,7 +1406,7 @@ xfs_attr_remove_iter(
 			state = dac->da_state;
 		}
 
-		/* fallthrough */
+		fallthrough;
 	case XFS_DAS_RMTBLK:
 		dac->dela_state = XFS_DAS_RMTBLK;
 
@@ -1441,7 +1441,7 @@ xfs_attr_remove_iter(
 			return -EAGAIN;
 		}
 
-		/* fallthrough */
+		fallthrough;
 	case XFS_DAS_RM_NAME:
 		/*
 		 * If we came here fresh from a transaction roll, reattach all
@@ -1469,7 +1469,7 @@ xfs_attr_remove_iter(
 			return -EAGAIN;
 		}
 
-		/* fallthrough */
+		fallthrough;
 	case XFS_DAS_RM_SHRINK:
 		/*
 		 * If the result is small enough, push it all into the inode.