Remove the resize flag and use the same code path for all callers.
This flag was added by commit 18f03166 to allow virStorageFileResize
use 'safezero' while preserving the behavior.

Explicitly return -2 when a fallback to a different method should
be done, to make the code path more obvious.

Fail immediately when ftruncate fails in the mmap method,
as we did before commit 18f03166.