[PATCH] erofs-utils: support --blobdev with block map

[Nithurshen]

Hi Ajay,

Thanks for looking into this, but this patch is redundant. I already
addressed this exact TODO item last month.

My initial patch was sent on March 7th. After discussing the
implementation with the maintainers, specifically addressing Yifan
Zhao's feedback regarding the 32-bit address limits for block map
entries and the necessary fix to reset m_deviceid in erofs_map_dev(),
I submitted a finalized v2 on March 9th.

Additionally, following Gao Xiang's guidance, I have already written
and submitted the corresponding test case to theexperimental-tests
branch as of March 19th.

I strongly suggest checking the recent mailing list archives or the
pending patch queue before picking up a TODO item to avoid stepping
on ongoing work and duplicating effort.

Regards,
Nithurshen