powerpc/mm: Free string after creating kmem cache

powerpc/mm: Free string after creating kmem cache

kmem_cache_create()->kmem_cache_create_memcg()->kstrdup() allocates new
space and copys name's content, so it is safe to free name memory after
calling kmem_cache_create(). Else kmemleak will report the below
warning:

unreferenced object 0xc0000000f9002160 (size 16):
  comm "swapper/0", pid 0, jiffies 4294892296 (age 1386.640s)
  hex dump (first 16 bytes):
    70 67 74 61 62 6c 65 2d 32 5e 39 00 de ad be ef  pgtable-2^9.....
  backtrace:
    [<c0000000004e03ec>] .kvasprintf+0x5c/0xa0
    [<c0000000004e045c>] .kasprintf+0x2c/0x50
    [<c00000000002e36c>] .pgtable_cache_add+0xac/0x100
    [<c00000000002e3e4>] .pgtable_cache_init+0x24/0x80
    [<c000000000c6c67c>] .start_kernel+0x228/0x4c8
    [<c000000000000594>] .start_here_common+0x24/0x90

Signed-off-by: Yanjiang Jin <yanjiang.jin@windriver.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>