mm/writeback: Add pageset_write_one

mm/writeback: Add pageset_write_one

Transform write_one_page() into pageset_write_one() and add a compatibility
wrapper.  Also move the declaration to pagemap.h as this is page cache
functionality that doesn't need to be used by the rest of the kernel.

Saves 58 bytes of kernel text.  While pageset_write_one() is 101 bytes
smaller than write_one_page(), the inlined call to page_pageset() expands
each caller.  There are fewer than ten callers so it doesn't seem worth
putting a wrapper in the core.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Howells <dhowells@redhat.com>