btrfs: add quota override flag to enable quota override for CAP_SYS_RESOURCE

This patch introduces the quota override flag to btrfs_fs_info, and a
change to quota limit checking code to temporarily allow for quota to be
overridden for processes with CAP_SYS_RESOURCE.

It's useful for administrative programs, such as log rotation, that may
need to temporarily use more disk space in order to free up a greater
amount of overall disk space without yielding more disk space to the
rest of userland.

Eventually, we may want to add the idea of an operator-specific quota,
operator reserved space, or something else to allow for administrative
override, but this is perhaps the simplest solution.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Reviewed-by: David Sterba <dsterba@suse.com>
[ minor changelog edits ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -716,6 +716,8 @@ struct btrfs_delayed_root;
 #define BTRFS_FS_BTREE_ERR			11
 #define BTRFS_FS_LOG1_ERR			12
 #define BTRFS_FS_LOG2_ERR			13
+#define BTRFS_FS_QUOTA_OVERRIDE			14
+
 /*
  * Indicate that a whole-filesystem exclusive operation is running
  * (device replace, resize, device add/delete, balance)

fs/btrfs/qgroup.c

@@ -2338,6 +2338,11 @@ static int qgroup_reserve(struct btrfs_root *root, u64 num_bytes, bool enforce)
 
 	if (num_bytes == 0)
 		return 0;
+
+	if (test_bit(BTRFS_FS_QUOTA_OVERRIDE, &fs_info->flags) &&
+	    capable(CAP_SYS_RESOURCE))
+		enforce = false;
+
 retry:
 	spin_lock(&fs_info->qgroup_lock);
 	quota_root = fs_info->quota_root;