btrfs: use struct btrfs_inode inside btrfs_remap_file_range_prep()

Use a struct btrfs_inode in btrfs_remap_file_range_prep() as it's an
internal helper, allowing to remove some use of BTRFS_I.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>