f2fs: Use a folio throughout f2fs_truncate_inode_blocks()

Use f2fs_get_node_folio() to get a folio and use it throughout.  Remove a
few calls to compound_head() and a reference to page->mapping.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>