bcachefs: Don't rely on implicit unsigned -> signed integer conversion

implicit integer conversion is a fertile source of bugs, and we really
would rather not have the min()/max() macros doing it implicitly.
bcachefs appears to be the only place in the kernel where this happens,
so let's fix it.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/alloc_background.h

@@ -166,7 +166,7 @@ static inline u64 alloc_lru_idx_fragmentation(struct bch_alloc_v4 a,
 	 * avoid overflowing LRU_TIME_BITS on a corrupted fs, when
 	 * bucket_sectors_dirty is (much) bigger than bucket_size
 	 */
-	u64 d = min(bch2_bucket_sectors_dirty(a),
+	u64 d = min_t(s64, bch2_bucket_sectors_dirty(a),
 		      ca->mi.bucket_size);
 
 	return div_u64(d * (1ULL << 31), ca->mi.bucket_size);