btrfs: relocation: Check cancel request after each data page read

btrfs: relocation: Check cancel request after each data page read

When relocating a data extents with large large data extents, we spend
most of our time in relocate_file_extent_cluster() at stage "moving data
extents":
 1)               |  btrfs_relocate_block_group [btrfs]() {
 1)               |    relocate_file_extent_cluster [btrfs]() {
 1) $ 6586769 us  |    }
 1) + 18.260 us   |    relocate_file_extent_cluster [btrfs]();
 1) + 15.770 us   |    relocate_file_extent_cluster [btrfs]();
 1) $ 8916340 us  |  }
 1)               |  btrfs_relocate_block_group [btrfs]() {
 1)               |    relocate_file_extent_cluster [btrfs]() {
 1) $ 11611586 us |    }
 1) + 16.930 us   |    relocate_file_extent_cluster [btrfs]();
 1) + 15.870 us   |    relocate_file_extent_cluster [btrfs]();
 1) $ 14986130 us |  }

So to make data relocation cancelling quicker, here add extra balance
cancelling check after each page read in relocate_file_extent_cluster().

Signed-off-by: Qu Wenruo <wqu@suse.com>
(cherry picked from commit 34bb874930d1d6fecec7927101041884fe695aa7)