UPSTREAM: f2fs: convert f2fs_write_begin() to use folio

Convert to use folio, so that we can get rid of 'page->index' to
prepare for removal of 'index' field in structure page [1].

[1] https://lore.kernel.org/all/Zp8fgUSIBGQ1TN0D@casper.infradead.org/



Cc: Matthew Wilcox <willy@infradead.org>
Change-Id: I83314899ea4ac7a678dc1fae470a842ba72f1df8
Signed-off-by: Chao Yu <chao@kernel.org>
Reviewed-by: Li Zetao <lizetao1@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit 12e61ce2)