* Rework page writing to combine page writes where possible. For now the pages

are required to be physically contiguos, which should be reworked to put them
  into seperate iovecs. Still this manages to combine a great deal of page
  writes into larger bursts already. Reduces the amount of IO requests being
  scheduled (and greatly benefits media where page wise writes are slow when
  they are accessed through a non-IOScheduler path, i.e. USB mass storage until
  that is properly implemented).
* Abstracted per page page writing tasks into a PageWriteWrapper class.
* Abstracted per transfer page writing tasks into PageWriteTransfer class which
  formerly was the PageWriterCallback.
* Use both classes from the PageWriterRun and from
  vm_page_write_modified_page_range to remove code duplication.
* Adjusted synchronous VMAnonymousCache::Write() to cope correctly with larger
  iovecs and more than one iovec. It assumed that there was exactly one page per
  vector previously.
* Introduced MaxPagesPerWrite() and MaxPagesPerAsyncWrite() to VMCache to allow
  a cache to specify restricitions. VMAnonymousCache does restrict the max pages
  to 1 for WriteAsync right now as I didn't feel like reworking that one to cope
  with non single page writes just yet.
* Pulled out PageWriteTransfer methods for better readability.
* Some typo fixes.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@33507 a95241bf-73f2-0310-859d-f6bbb57e9c96

src/system/kernel/cache/file_cache.cpp

@@ -363,7 +363,7 @@ read_pages_and_clear_partial(file_cache_ref* ref, void* cookie, off_t offset,
 	pages needed to fulfill that request. This function is called by cache_io().
 	It can only handle a certain amount of bytes, and the caller must make
 	sure that it matches that criterion.
-	The cache_ref lock must be hold when calling this function; during
+	The cache_ref lock must be held when calling this function; during
 	operation it will unlock the cache, though.
 */
 static status_t