Btrfs: split up __extent_writepage to lower stack usage

__extent_writepage has two unrelated parts.  First it does the delayed
allocation dance and second it does the mapping and IO for the page
we're actually writing.

This splits it up into those two parts so the stack from one doesn't
impact the stack from the other.
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/inode.c

@@ -125,7 +125,7 @@ static int btrfs_init_inode_security(struct btrfs_trans_handle *trans,
  * the btree.  The caller should have done a btrfs_drop_extents so that
  * no overlapping inline items exist in the btree
  */
-static noinline int insert_inline_extent(struct btrfs_trans_handle *trans,
+static int insert_inline_extent(struct btrfs_trans_handle *trans,
 				struct btrfs_path *path, int extent_inserted,
 				struct btrfs_root *root, struct inode *inode,
 				u64 start, size_t size, size_t compressed_size,