bcachefs: Break up io.c

More reorganization, this splits up io.c into
 - io_read.c
 - io_misc.c - fallocate, fpunch, truncate
 - io_write.c

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/move.c

@@ -14,7 +14,8 @@
 #include "errcode.h"
 #include "error.h"
 #include "inode.h"
-#include "io.h"
+#include "io_read.h"
+#include "io_write.h"
 #include "journal_reclaim.h"
 #include "keylist.h"
 #include "move.h"