Fix 3.18.8 breakage of UBI devices with EOF marker (e.g. WNDR4300)

This commit re-adds a patch from 3.14 that is required for UBI block
devices with an EOF marker to be successfully mounted.

Signed-off-by: Oliver Smith <oliver@8.c.9.b.0.7.4.0.1.0.0.2.ip6.arpa>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44658 3c298f89-4303-0410-b956-a3cf2f4a3e73

target/linux/generic/patches-3.18/490-mtd-ubi-add-EOF-marker-support.patch

@@ -0,0 +1,51 @@
+--- a/drivers/mtd/ubi/attach.c
++++ b/drivers/mtd/ubi/attach.c
+@@ -800,6 +800,13 @@ out_unlock:
+ 	return err;
+ }
+ 
++static bool ec_hdr_has_eof(struct ubi_ec_hdr *ech)
++{
++	return ech->padding1[0] == 'E' &&
++	       ech->padding1[1] == 'O' &&
++	       ech->padding1[2] == 'F';
++}
++
+ /**
+  * scan_peb - scan and process UBI headers of a PEB.
+  * @ubi: UBI device description object
+@@ -830,9 +837,21 @@ static int scan_peb(struct ubi_device *u
+ 		return 0;
+ 	}
+ 
+-	err = ubi_io_read_ec_hdr(ubi, pnum, ech, 0);
+-	if (err < 0)
+-		return err;
++	if (!ai->eof_found) {
++		err = ubi_io_read_ec_hdr(ubi, pnum, ech, 0);
++		if (err < 0)
++			return err;
++
++		if (ec_hdr_has_eof(ech)) {
++			ubi_msg("EOF marker found, PEBs from %d will be erased",
++				pnum);
++			ai->eof_found = true;
++		}
++	}
++
++	if (ai->eof_found)
++		err = UBI_IO_FF_BITFLIPS;
++
+ 	switch (err) {
+ 	case 0:
+ 		break;
+--- a/drivers/mtd/ubi/ubi.h
++++ b/drivers/mtd/ubi/ubi.h
+@@ -701,6 +701,7 @@ struct ubi_attach_info {
+ 	int mean_ec;
+ 	uint64_t ec_sum;
+ 	int ec_count;
++	bool eof_found;
+ 	struct kmem_cache *aeb_slab_cache;
+ };
+