btrfs: replace remaining do_div calls with div_u64 variants

Switch to div_u64_rem that does type checking and has more obvious
semantics than do_div.

Signed-off-by: David Sterba <dsterba@suse.cz>

diff --cc fs/btrfs/extent-tree.c
Simple merge

diff --cc fs/btrfs/free-space-cache.c
Simple merge

diff --cc fs/btrfs/scrub.c
Simple merge

diff --cc fs/btrfs/volumes.c
Simple merge