add_to_filemap race condition

[psilabs-dev]

Caught by a test.

An archive can exist here:

LANraragi/lib/Shinobu.pm

Line 186 in d79dd8f

my $cnt = 0;

But not here:

LANraragi/lib/Shinobu.pm

Line 196 in d79dd8f

eval { $id = compute_id($file); };

triggering a die.

More generally, there's no guarantee of file existence made to compute_id (cnt loop only sleeps at most 5s, which can lead file to be deleted elsewhere). We should probably demote the error logs to warning here, or lock the resource and ensure shinobu can read the file without a delete API interfering.