[NaCl SDK] nacl_io: Fix FuseFs.{read,write}

[NaCl SDK] nacl_io: Fix FuseFs.{read,write}

I misinterpreted the docs as meaning that read() and write() should
always return the number of bytes requested.

What it actually says is that the functions should always return the
number of bytes requested _unless_ the EOF is reached or there is an
error.

BUG=411557
R=sbc@chromium.org

Committed: https://crrev.com/49b78e743721747e1b41ad055c0d9999b65a1510
Cr-Commit-Position: refs/heads/master@{#294018}

[Sam Clegg, 2014-09-08 23:33:54]

lgtm

[binji, 2014-09-09 20:45:52]

The CQ bit was checked by binji@chromium.org

[commit-bot: I haz the power, 2014-09-09 20:58:28]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/binji@chromium.org/549353003/1

[commit-bot: I haz the power, 2014-09-09 21:41:43]

Committed patchset #1 (id:1) as 25409b974340c9875997d2aa68bec876982e86fd

[commit-bot: I haz the power, 2014-09-10 03:55:57]

Patchset 1 (id:??) landed as
https://crrev.com/49b78e743721747e1b41ad055c0d9999b65a1510
Cr-Commit-Position: refs/heads/master@{#294018}