prelloc: warn and continue if fallocate(2) is not supported
prelloc: warn and continue if fallocate(2) is not supported

This makes gocryptfs work at all on ZFS.
See https://github.com/rfjakob/gocryptfs/issues/22 .

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