In cluster_pages_for_defrag(), we have complex code block inside one
for() loop.

The code block is to prepare one page for defrag, this will ensure:
- The page is locked and set up properly
- No ordered extent exists in the page range
- The page is uptodate
- The writeback has finished

This behavior is pretty common and will be reused by later defrag
rework.

So extract the code into its own helper, defrag_prepare_one_page(), for
later usage, and cleanup the code by a little.

Since we're here, also make the page check to be subpage compatible,
which means we will also check page::private, not only page->mapping.

Signed-off-by: Qu Wenruo <redacted>
---
 fs/btrfs/ioctl.c | 151 +++++++++++++++++++++++++++--------------------
 1 file changed, 86 insertions(+), 65 deletions(-)

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 05af6f5ff3ff..a06ceb8fdf28 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c

@@ -1143,6 +1143,89 @@ static int should_defrag_range(struct inode *inode, u64 start, u32 thresh,
 	return ret;
 }
 
+/*
+ * Prepare one page to be defragged.
+ *
+ * This will ensure:
+ * - Returned page is locked and has been set up properly
+ * - No ordered extent exists in the page
+ * - The page is uptodate
+ * - The writeback has finished
+ */
+static struct page *defrag_prepare_one_page(struct btrfs_inode *inode,
+					    unsigned long index)
+{
+	struct address_space *mapping = inode->vfs_inode.i_mapping;
+	gfp_t mask = btrfs_alloc_write_mask(mapping);
+	u64 page_start = index << PAGE_SHIFT;