swap file broken with ext4 fast-commit

[Andrea Righi <andrea.righi@canonical.com>]

I'm getting the following error if I try to create and activate a swap
file defined on an ext4 filesystem:

 [   34.406479] swapon: file is not committed

The swap file is created in the root filesystem (ext4 mounted with the
following options):

$ grep " / " /proc/mounts
/dev/vda1 / ext4 rw,relatime 0 0

No matter how long I wait or how many times I run sync, I'm still
getting the same error and the swap file is never activated.

A git bisect shows that this issue has been introduced by the following
commit:

 aa75f4d3daae ("ext4: main fast-commit commit path")

Simple test case to reproduce the problem:

 # fallocate -l 8G /swapfile
 # chmod 0600 /swapfile
 # mkswap /swapfile
 # swapon /swapfile

Maybe we're missing to mark the inode as clean somewhere, even if the
transation is committed to the journal?

Thanks,
-Andrea