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>