ubifs: Correctly initialize c->min_log_bytes

Currently on a freshly mounted UBIFS, c->min_log_bytes is 0.
This can lead to a log overrun and make commits fail.

Recent kernels will report the following assert:
UBIFS assert failed: c->lhead_lnum != c->ltail_lnum, in fs/ubifs/log.c:412

c->min_log_bytes can have two states, 0 and c->leb_size.
It controls how much bytes of the log area are reserved for non-bud
nodes such as commit nodes.

After a commit it has to be set to c->leb_size such that we have always
enough space for a commit. While a commit runs it can be 0 to make the
remaining bytes of the log available to writers.

Having it set to 0 right after mount is wrong since no space for commits
is reserved.

Fixes: 1e51764a ("UBIFS: add new flash file system")
Reported-and-tested-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Richard Weinberger <richard@nod.at>

fs/ubifs/super.c

@@ -609,6 +609,10 @@ static int init_constants_early(struct ubifs_info *c)
 	c->max_bu_buf_len = UBIFS_MAX_BULK_READ * UBIFS_MAX_DATA_NODE_SZ;
 	if (c->max_bu_buf_len > c->leb_size)
 		c->max_bu_buf_len = c->leb_size;
+
+	/* Log is ready, preserve one LEB for commits. */
+	c->min_log_bytes = c->leb_size;
+
 	return 0;
 }