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
diff --git a/fs/sdcardfs/inode.c b/fs/sdcardfs/inode.c
index 683b492..4a971e2 100644
--- a/fs/sdcardfs/inode.c
+++ b/fs/sdcardfs/inode.c
@@ -34,10 +34,14 @@
 	if (!cred)
 		return NULL;
 
-	if (data->under_obb)
-		uid = AID_MEDIA_OBB;
-	else
-		uid = multiuser_get_uid(data->userid, sbi->options.fs_low_uid);
+	if (sbi->options.gid_derivation) {
+		if (data->under_obb)
+			uid = AID_MEDIA_OBB;
+		else
+			uid = multiuser_get_uid(data->userid, sbi->options.fs_low_uid);
+	} else {
+		uid = sbi->options.fs_low_uid;
+	}
 	cred->fsuid = make_kuid(&init_user_ns, uid);
 	cred->fsgid = make_kgid(&init_user_ns, sbi->options.fs_low_gid);