Teach mdnblocks() not to create zero-length files.

Teach mdnblocks() not to create zero-length files.

It's entirely surprising that mdnblocks() has the side effect of
creating new files on disk, so let's make it not do that.  One
consequence of the old behavior is that, if running on a damaged
cluster that is missing a file, mdnblocks() can recreate the file
and allow a subsequent _mdfd_getseg() for a higher segment to succeed.
This happens because, while mdnblocks() stops when it finds a segment
that is shorter than 1GB, _mdfd_getseg() has no such check, and thus
the empty file created by mdnblocks() can allow it to continue its
traversal and find higher-numbered segments which remain.

It might be a good idea for _mdfd_getseg() to actually verify that
each segment it finds is exactly 1GB before proceeding to the next
one, but that would involve some additional system calls, so for
now I'm just doing this much.

Patch by me, per off-list analysis by Kevin Grittner and Rahila Syed.
Review by Andres Freund.