Introduce vnode op supports_operation(), fix devfs_io()

devfs_io() can't fall back to calling vfs_synchronous_io(), if the
device driver doesn't support handling requests asynchronously. The
presence of the io() hook leads the VFS (do_iterative_fd_io()) to
believe that asynchronous handling is supported and set a
finished-callback on the request which calls the io() hook to start the
next chunk. Thus, instead of iterating through the request in a loop
the iteration happens recursively. For sufficiently fragmented requests
the stack may overflow (ticket #9900).

* Introduce a new vnode operation supports_operation(). It can be called
  by the VFS to determine whether a present hook is actually currently
  supported for a given vnode.
* devfs: implement the new hook and remove the fallback handling in
  devfs_io().
* vfs_request_io.cpp: use the new hook to determine whether the io()
  hook is really supported.

headers/os/drivers/fs_interface.h

@@ -45,6 +45,11 @@ struct file_io_vec {
 #define B_VNODE_DONT_CREATE_SPECIAL_SUB_NODE	0x02
 
 
+enum BVnodeOperation {
+	B_VNODE_OPERATION_IO	= 0
+};
+
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -250,6 +255,10 @@ struct fs_vnode_ops {
 				const struct flock* lock, bool wait);
 	status_t (*release_lock)(fs_volume* volume, fs_vnode* vnode, void* cookie,
 				const struct flock* lock);
+
+	/* supported vnode operations info */
+	bool (*supports_operation)(fs_volume* volume, fs_vnode* vnode,
+				enum BVnodeOperation operation);
 };
 
 struct file_system_module_info {