powerpc/fadump: Fix build error with CONFIG_PRESERVE_FA_DUMP=y

skiroot_defconfig fails:

arch/powerpc/kernel/fadump.c:48:17: error: ‘cpus_in_fadump’ defined but not used
   48 | static atomic_t cpus_in_fadump;

Fix it by moving the definition into the #ifdef where it's used.

Fixes: ba608c4f ("powerpc/fadump: fix race between pstore write and fadump crash trigger")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200727070341.595634-1-mpe@ellerman.id.au

arch/powerpc/kernel/fadump.c

@@ -45,10 +45,12 @@ static struct fw_dump fw_dump;
 static void __init fadump_reserve_crash_area(u64 base);
 
 struct kobject *fadump_kobj;
-static atomic_t cpus_in_fadump;
 
 #ifndef CONFIG_PRESERVE_FA_DUMP
+
+static atomic_t cpus_in_fadump;
 static DEFINE_MUTEX(fadump_mutex);
+
 struct fadump_mrange_info crash_mrange_info = { "crash", NULL, 0, 0, 0, false };
 
 #define RESERVED_RNGS_SZ	16384 /* 16K - 128 entries */