orangefs: Simplify bvec setup in orangefs_writepages_work()

orangefs: Simplify bvec setup in orangefs_writepages_work()

This produces a bvec which is slightly different as the last page is added
in its entirety rather than only the portion which is being written back.
However we don't use this information anywhere; the iovec has its own
length parameter.

Signed-off-by: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Link: https://lore.kernel.org/r/20250305204734.1475264-9-willy@infradead.org
Tested-by: Mike Marshall <hubcap@omnibond.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>