qcow2: Return the original error code in qcow2_co_pwrite_zeroes()

qcow2: Return the original error code in qcow2_co_pwrite_zeroes()

This function checks the current status of a (sub)cluster in order to
see if an unaligned 'write zeroes' request can be done efficiently by
simply updating the L2 metadata and without having to write actual
zeroes to disk.

If the situation does not allow using the fast path then the function
returns -ENOTSUP and the caller falls back to writing zeroes.

If can happen however that the aforementioned check returns an actual
error code so in this case we should pass it to the caller.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-Id: <20200909123739.719-1-berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>