LINUX: Use afs_convert_code in afs_notify_change

LINUX: Use afs_convert_code in afs_notify_change

afs_notify_change currently just returns "-code". This can cause a
panic if the error code is negative, since we will return a positive
error code, which may get interpreted as a valid pointer value in
higher levels.

Specifically, if we hit afs_notify_change via something like this code
path:

do_sys_open -> do_filp_open -> open_namei -> may_open -> do_truncate
-> notify_change -> afs_notify_change (via inode->i_op->setattr)

the positive error value will be interpreted by do_sys_open as a valid
'struct file' pointer, and will be dereferenced.

So pass the return value through afs_convert_code, like all of the
other vnode ops, so we ensure we return an error properly.

Change-Id: I941cc2d77d5a96cf86dfe133d30af4ccf6e9f2cb
Reviewed-on: http://gerrit.openafs.org/6776
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>