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.

docs/user/drivers/fs_interface.dox

@@ -92,6 +92,16 @@
 */
 
 
+/*!
+	\enum BVnodeOperation
+	Enumeration type for specifying vnode operations.
+*/
+
+/*!
+	\var BVnodeOperation B_VNODE_OPERATION_IO
+	Refers to the fs_vnode_ops::io operation.
+*/
+
 ///// file_system_module_info /////
 
 
@@ -1698,6 +1708,31 @@
 	\brief TODO: Document!
 */
 
+/* TODO: test_lock(), acquire_lock(), release_lock() */
+
+/*!
+	\fn bool (*fs_vnode_ops::supports_operation)(fs_volume* volume,
+			fs_vnode* vnode, BVnodeOperation operation)
+	\brief Returns whether the specified vnode operation is supported for the
+		specified vnode.
+
+	For most vnode operations the FS can simply indicate that it doesn't support
+	it by setting the respective attribute in the fs_vnode_ops structure to
+	\c NULL. For certain nodes the support of an operation may change over the
+	life time of the node. While in most cases the implementation of the
+	operation can then simply return an error (\c B_NOT_SUPPORTED), for some
+	operations, however, the VFS has to know beforehand whether to call it at
+	all or resort to a different handling. It calls this hook only for
+	operations that aren't set to \c NULL in the fs_vnode_ops structure. If this
+	hook is not present, the VFS behaves as if it was present always returned
+	\c true.
+
+	\param volume The volume object.
+	\param vnode The node object.
+	\param operation The enum value identifying the operation.
+	\return \c true, if the specified operation is currently supported, \c false
+		otherwise.
+*/
 
 //! @}