UBI: introduce volume refcounting

Add ref_count field to UBI volumes and remove weired "vol->removed"
field. This way things are better understandable and we do not have
to do whold show_attr operation under spinlock.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
diff --git a/drivers/mtd/ubi/eba.c b/drivers/mtd/ubi/eba.c
index 2ff3492..84f7dc9 100644
--- a/drivers/mtd/ubi/eba.c
+++ b/drivers/mtd/ubi/eba.c
@@ -301,6 +301,8 @@
 {
 	int err, pnum, vol_id = vol->vol_id;
 
+	ubi_assert(vol->ref_count > 0);
+
 	if (ubi->ro_mode)
 		return -EROFS;
 
@@ -349,6 +351,8 @@
 	struct ubi_vid_hdr *vid_hdr;
 	uint32_t uninitialized_var(crc);
 
+	ubi_assert(vol->ref_count > 0);
+
 	err = leb_read_lock(ubi, vol_id, lnum);
 	if (err)
 		return err;
@@ -572,6 +576,8 @@
 	int err, pnum, tries = 0, vol_id = vol->vol_id;
 	struct ubi_vid_hdr *vid_hdr;
 
+	ubi_assert(vol->ref_count > 0);
+
 	if (ubi->ro_mode)
 		return -EROFS;
 
@@ -705,6 +711,8 @@
 	struct ubi_vid_hdr *vid_hdr;
 	uint32_t crc;
 
+	ubi_assert(vol->ref_count > 0);
+
 	if (ubi->ro_mode)
 		return -EROFS;
 
@@ -819,6 +827,8 @@
 	struct ubi_vid_hdr *vid_hdr;
 	uint32_t crc;
 
+	ubi_assert(vol->ref_count > 0);
+
 	if (ubi->ro_mode)
 		return -EROFS;