powerpc/rtas_flash: Free kmem upon module exit

powerpc/rtas_flash: Free kmem upon module exit

Memory allocated to rtas_firmware_flash_list in rtas_flash_write
is not freed during module exit. We hit below call trace if we
unload rtas_flash module after loading new firmware image and
before rebooting the system.

Call trace:
----------
Feb  6 08:42:10 eagle3 kernel: kmem_cache_destroy rtas_flash_cache: Slab cache still has objects
Feb  6 08:42:10 eagle3 kernel: Call Trace:
Feb  6 08:42:10 eagle3 kernel: [c00000001c303b40] [c000000000014940] .show_stack+0x70/0x1c0 (unreliable)
Feb  6 08:42:10 eagle3 kernel: [c00000001c303bf0] [c000000000199bec] .kmem_cache_destroy+0x15c/0x170
Feb  6 08:42:10 eagle3 kernel: [c00000001c303c90] [d000000006fa1208] .rtas_flash_cleanup+0x3c/0x80 [rtas_flash]
Feb  6 08:42:10 eagle3 kernel: [c00000001c303d20] [c0000000000f8970] .SyS_delete_module+0x1d0/0x2e0
Feb  6 08:42:10 eagle3 kernel: [c00000001c303e30] [c000000000009954] syscall_exit+0x0/0x94

This patch frees rtas_firmware_flash_list during module exit.

Signed-off-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>