ANDROID: sdcardfs: Enable modular sdcardfs

Export the following symbols:

- copy_fs_struct
- free_fs_struct
- security_path_chown
- set_fs_pwd
- vfs_read
- vfs_write

These are needed to build sdcardfs as a module.

Test: HiKey/X15 + Pie + android-mainline,
      and HiKey + AOSP Maser + android-mainline,
      directories under /sdcard created,
      output of mount is right,
      CTS test collecting device infor works

Bug: 35142419
Change-Id: If6e14f0b3bdc858a9f684e6c209927a9232091f0
Signed-off-by: Daniel Rosenberg <drosen@google.com>
Signed-off-by: Guenter Roeck <groeck@chromium.org>
[astrachan: Folded the following changes into this patch:
            e19f69662df5 ("ANDROID: Revert "fs: unexport vfs_read and vfs_write"")
            17071a8e1e7d ("ANDROID: fs: Export free_fs_struct and set_fs_pwd")
            2e9a639597cd ("ANDROID: export security_path_chown")]
Signed-off-by: Alistair Strachan <astrachan@google.com>
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>