bcachefs: Fix debug build in userspace

This fixes some compiler warnings that only trigger in userspace - dead
code, a maybe uninitialed variable, a maybe null ptr passed to printk.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/btree_io.c

@@ -560,7 +560,8 @@ enum btree_validate_ret {
 									\
 	switch (write) {						\
 	case READ:							\
-		bch_err(c, "%s", _buf2);				\
+		if (_buf2)						\
+			bch_err(c, "%s", _buf2);			\
 									\
 		switch (type) {						\
 		case BTREE_ERR_FIXABLE:					\

fs/bcachefs/dirent.c

@@ -230,7 +230,7 @@ int bch2_dirent_rename(struct btree_trans *trans,
 {
 	struct btree_iter src_iter = { NULL };
 	struct btree_iter dst_iter = { NULL };
-	struct bkey_s_c old_src, old_dst;
+	struct bkey_s_c old_src, old_dst = bkey_s_c_null;
 	struct bkey_i_dirent *new_src = NULL, *new_dst = NULL;
 	struct bpos dst_pos =
 		POS(dst_dir.inum, bch2_dirent_hash(dst_hash, dst_name));

fs/bcachefs/inode.c

@@ -24,16 +24,6 @@ const char * const bch2_inode_opts[] = {
 };
 
 static const u8 byte_table[8] = { 1, 2, 3, 4, 6, 8, 10, 13 };
-static const u8 bits_table[8] = {
-	1  * 8 - 1,
-	2  * 8 - 2,
-	3  * 8 - 3,
-	4  * 8 - 4,
-	6  * 8 - 5,
-	8  * 8 - 6,
-	10 * 8 - 7,
-	13 * 8 - 8,
-};
 
 static int inode_decode_field(const u8 *in, const u8 *end,
 			      u64 out[2], unsigned *out_bits)