fusefrontend: log prealloc failures at Info level
fusefrontend: log prealloc failures at Info level

If the underlying filesystem is full, it is normal get ENOSPC here.
Log at Info level instead of Warning.

Fixes xfstests generic/015 and generic/027, which complained about
the extra output.

rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs