docs: filesystems: vfs: correct sync_mode flag names

Change the nonexistent flag names WBC_SYNC_ALL and WBC_SYNC_NONE to
WB_SYNC_ALL and WB_SYNC_NONE, respectively, as used in the code with
wbc->sync_mode.

Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>

Link: https://lore.kernel.org/r/1595791341-13209-1-git-send-email-Julia.Lawall@inria.fr
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/vfs.rst

@@ -766,9 +766,9 @@ cache in your filesystem.  The following members are defined:
 
 ``writepages``
 	called by the VM to write out pages associated with the
-	address_space object.  If wbc->sync_mode is WBC_SYNC_ALL, then
+	address_space object.  If wbc->sync_mode is WB_SYNC_ALL, then
 	the writeback_control will specify a range of pages that must be
-	written out.  If it is WBC_SYNC_NONE, then a nr_to_write is
+	written out.  If it is WB_SYNC_NONE, then a nr_to_write is
 	given and that many pages should be written if possible.  If no
 	->writepages is given, then mpage_writepages is used instead.
 	This will choose pages from the address space that are tagged as