ANDROID: Sdcardfs: Move gid derivation under flag

This moves the code to adjust the gid/uid of lower filesystem
files under the mount flag derive_gid.

Signed-off-by: Daniel Rosenberg <drosen@google.com>
Change-Id: I44eaad4ef67c7fcfda3b6ea3502afab94442610c
Bug: 63245673
Git-commit: 53a08829
Git-repo: https://android.googlesource.com/kernel/common/


Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>