Linux 5.2 compat: Directly call wait_on_page_bit()

wait_on_page_writeback() was made GPL only in torvalds/linux@19343b5bdd.

Directly call wait_on_page_bit() without using wait_on_page_writeback()
interface, given zfs_putpage() is the only caller for now.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@osnexus.com>
Closes openzfs#8794

module/os/linux/zfs/zfs_vnops.c

@@ -4526,8 +4526,10 @@ zfs_putpage(struct inode *ip, struct page *pp, struct writeback_control *wbc)
 		unlock_page(pp);
 		rangelock_exit(lr);
 
-		if (wbc->sync_mode != WB_SYNC_NONE)
-			wait_on_page_writeback(pp);
+		if (wbc->sync_mode != WB_SYNC_NONE) {
+			if (PageWriteback(pp))
+				wait_on_page_bit(pp, PG_writeback);
+		}
 
 		ZFS_EXIT(zfsvfs);
 		return (0);