btrfs: introduce pending action: commit

In some contexts, like in sysfs handlers, we don't want to trigger a
transaction commit. It's a heavy operation, we don't know what external
locks may be taken. Instead, make it possible to finish the operation
through sync syscall or SYNC_FS ioctl.
Signed-off-by: David Sterba <dsterba@suse.cz>

fs/btrfs/ctree.h

@@ -2132,6 +2132,7 @@ struct btrfs_ioctl_defrag_range_args {
 
 #define BTRFS_PENDING_SET_INODE_MAP_CACHE	(0)
 #define BTRFS_PENDING_CLEAR_INODE_MAP_CACHE	(1)
+#define BTRFS_PENDING_COMMIT			(2)
 
 #define btrfs_test_pending(info, opt)	\
 	test_bit(BTRFS_PENDING_##opt, &(info)->pending_changes)

fs/btrfs/transaction.c

@@ -2036,6 +2036,11 @@ void btrfs_apply_pending_changes(struct btrfs_fs_info *fs_info)
 		btrfs_clear_opt(fs_info->mount_opt, INODE_MAP_CACHE);
 	prev &= ~bit;
 
+	bit = 1 << BTRFS_PENDING_COMMIT;
+	if (prev & bit)
+		btrfs_debug(fs_info, "pending commit done");
+	prev &= ~bit;
+
 	if (prev)
 		btrfs_warn(fs_info,
 			"unknown pending changes left 0x%lx, ignoring", prev);