ubifs: remove unnecessary check in ubifs_log_start_commit

In ubifs_log_start_commit, the value of c->lhead_offs is zero or set
to zero by code bellow.

	/* Switch to the next log LEB */
	if (c->lhead_offs) {
		c->lhead_lnum = ubifs_next_log_lnum(c, c->lhead_lnum);
		ubifs_assert(c->lhead_lnum != c->ltail_lnum);
		c->lhead_offs = 0;
	}

The value of 'len' can not exceed 'max_len' which assigned value by
code bellow.

	max_len = UBIFS_CS_NODE_SZ + c->jhead_cnt * UBIFS_REF_NODE_SZ;

The value of c->lhead_offs changed by code bellow and cannot exceed
'max_len'.

	c->lhead_offs += len;
	if (c->lhead_offs == c->leb_size) {
		c->lhead_lnum = ubifs_next_log_lnum(c, c->lhead_lnum);
		c->lhead_offs = 0;
	}

Usually, the size of PEB is between 64KB and 256KB. So the value of
c->lhead_offs is far less than c->leb_size. The check
'if (c->lhead_offs == c->leb_size)' could never to be true.

Signed-off-by: Liu Song <liu.song11@zte.com.cn>
Reviewed-by: Jiang Biao <jiang.biao2@zte.com.cn>
Signed-off-by: Richard Weinberger <richard@nod.at>