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/1d4f72f7fee285b2ddf4bf62b0ac0fd89def5417.1728575379.git.naohiro.aota@wdc.com/

Link: https://lore.kernel.org/all/20241016134919.GO1609@suse.cz/
Acked-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: David Sterba <dsterba@suse.com>

arch/mips/kernel/cmpxchg.c

@@ -102,3 +102,4 @@ unsigned long __cmpxchg_small(volatile void *ptr, unsigned long old,
 			return old;
 	}
 }
+EXPORT_SYMBOL(__cmpxchg_small);