file-posix: Fix no-op bdrv_truncate() with falloc preallocation

file-posix: Fix no-op bdrv_truncate() with falloc preallocation

If bdrv_truncate() is called, but the requested size is the same as
before, don't call posix_fallocate(), which returns -EINVAL for length
zero and would therefore make bdrv_truncate() fail.

The problem can be triggered by creating a zero-sized raw image with
'falloc' preallocation mode.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>