[XFS] Remove unused wbc parameter from xfs_start_page_writeback()

SGI-PV: 981498
SGI-Modid: xfs-linux-melb:xfs-kern:31057a

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>