fs: Don't hide error from prepare_fs

If fchroot() succeeds the further failures don't get
noticed by caller.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>
fs: Opening FE-s after fchdir doesn't work

It uses absolute file names, so any open-s should happen _before_
we change tasks' root.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>