ext4: Remove "extents" mount option

This mount option is largely superfluous, and in fact the way it was
implemented was buggy; if a filesystem which did not have the extents
feature flag was mounted -o extents, the filesystem would attempt to
create and use extents-based file even though the extents feature flag
was not eabled.  The simplest thing to do is to nuke the mount option
entirely.  It's not all that useful to force the non-creation of new
extent-based files if the filesystem can support it.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/fs/ext4/migrate.c b/fs/ext4/migrate.c
index e7cd488..734abca 100644
--- a/fs/ext4/migrate.c
+++ b/fs/ext4/migrate.c
@@ -459,13 +459,13 @@
 	struct list_blocks_struct lb;
 	unsigned long max_entries;
 
-	if (!test_opt(inode->i_sb, EXTENTS))
-		/*
-		 * if mounted with noextents we don't allow the migrate
-		 */
-		return -EINVAL;
-
-	if ((EXT4_I(inode)->i_flags & EXT4_EXTENTS_FL))
+	/*
+	 * If the filesystem does not support extents, or the inode
+	 * already is extent-based, error out.
+	 */
+	if (!EXT4_HAS_INCOMPAT_FEATURE(inode->i_sb,
+				       EXT4_FEATURE_INCOMPAT_EXTENTS) ||
+	    (EXT4_I(inode)->i_flags & EXT4_EXTENTS_FL))
 		return -EINVAL;
 
 	if (S_ISLNK(inode->i_mode) && inode->i_blocks == 0)