MIPS: export __cmpxchg_small()

Export the symbol __cmpxchg_small() for btrfs.ko that uses it to store
blk_status_t, which is u8. Reported by LKP:

>> ERROR: modpost: "__cmpxchg_small" [fs/btrfs/btrfs.ko] undefined!

Patch using the cmpxchg() https://lore.kernel.org/linux-btrfs/1d4f[e-mail]/

Link: https://lore.kernel.org/all/[e-mail]/
Acked-by: Thomas Bogendoerfer <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

arch/mips/kernel/cmpxchg.c

···
 			return old;
 	}
 }
+EXPORT_SYMBOL(__cmpxchg_small);