f2fs: Use folio_next_pos()

f2fs: Use folio_next_pos()

This is one instruction more efficient than open-coding folio_pos() +
folio_size().  It's the equivalent of (x + y) << z rather than
x << z + y << z.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://patch.msgid.link/20251024170822.1427218-6-willy@infradead.org
Reviewed-by: Chao Yu <chao@kernel.org>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: Chao Yu <chao@kernel.org>
Cc: linux-f2fs-devel@lists.sourceforge.net
Signed-off-by: Christian Brauner <brauner@kernel.org>