mtd: slram: remove failed entries from the device list

In the Linux kernel, the following vulnerability has been resolved: mtd: slram: remove failed entries from the device list register_device() links a new slram_mtdlist entry before allocating all of the state needed by the entry. If a later allocation, memremap(), or mtd_device_register() fails, the partially initialized entry remains on the global list. A later cleanup can then dereference or free invalid state from that failed entry. Unwind the partially initialized entry and clear the list tail on each failure path after the entry has been linked.