btrfs-progs: subvolume delete: enhance warning message when deleting default subvolume

There are more possible reason for EPERM when subvolume is deleted,
deleting the default was not mentioned but can easily happen in
practice.

Signed-off-by: David Sterba <dsterba@suse.com>

Author: kdave

Documentation/btrfs-subvolume.rst

@@ -74,8 +74,8 @@ create [options] [<dest>/]<name> [[<dest2>/]<name2> ...]
 delete [options] [<subvolume> [<subvolume>...]], delete -i|--subvolid <subvolid> <path>
         Delete the subvolume(s) from the filesystem.
 
-        If *subvolume* is not a subvolume, btrfs returns an error but continues if
-        there are more arguments to process.
+        If *subvolume* is not a subvolume, the command returns an error but
+        continues if there are more arguments to process.
 
         If *--subvolid* is used, *path* must point to a btrfs filesystem. See
         :ref:`btrfs subvolume list<man-subvolume-list>` or
@@ -86,10 +86,10 @@ delete [options] [<subvolume> [<subvolume>...]], delete -i|--subvolid <subvolid>
         removed later in the background. The command returns immediately. See
         :ref:`btrfs subvolume sync<man-subvolume-sync>` how to wait until the subvolume gets completely removed.
 
-        The deletion does not involve full transaction commit by default due to
+        The deletion does not involve a full transaction commit by default due to
         performance reasons.  As a consequence, the subvolume may appear again after a
         crash.  Use one of the *--commit* options to wait until the operation is
-        safely stored on the device.
+        safely stored on the device before the command returns.
 
         Deleting subvolume needs sufficient permissions, by default the owner
         cannot delete it unless it's enabled by a mount option

cmds/subvolume.c

@@ -562,7 +562,7 @@ static int cmd_subvolume_delete(const struct cmd_struct *cmd, int argc, char **a
 
 		error_btrfs_util(err);
 		if (saved_errno == EPERM)
-			warning("deletion failed with EPERM, you don't have permissions or send may be in progress");
+			warning("deletion failed with EPERM, you don't have permissions or send may be in progress or the subvolume is set as default");
 		ret = 1;
 		goto out;
 	}