fs: tftp: Fix writing files to tftp

Copying files to tftp is broken since:

| commit d4f5bb1e011ac653a167031554f0ac9e028e9e36
| Author: Sascha Hauer <s.hauer@pengutronix.de>
| Date:   Sat Sep 28 13:12:50 2013 +0200
|
|     copy_file: Add missing O_TRUNC
|
|     Without it, when copying a smaller file over a larger file the
|     resulting file still has the remaining space from the larger file.
|
|     Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Since this commit copy_file passes O_TRUNC to open(). This results in the
fs layer calling fsdrv->truncate. tftp returns -ENOSYS here and open returns
with an error. To fix this return 0 for the truncate callback in tftp. Also
enforce the O_TRUNC flag when opening a file for writing on tftp as the files
are always truncated on tftp anyway.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reported-by: andreas.willig@rafi.de