ANDROID: sdcardfs: fix export symbol types

In commit c0dbfed5 ("ANDROID: sdcardfs: Enable modular sdcardfs") a
number of core vfs functions were exported, but they needed to be set as
_GPL symbols.

Fix this up by properly setting the export type of these symbols.

Bug: 35142419
Cc: Daniel Rosenberg <drosen@google.com>
Cc: Alistair Strachan <astrachan@google.com>
Cc: Yongqin Liu <yongqin.liu@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I3169efeb5b525c536958b1354318cf0fcd07e6e1