Btrfs: reuse cmp workspace in EXTENT_SAME ioctl

We support big dedup requests by splitting range to smaller parts, and
call dedupe logic on each of them.

Instead of repeated allocation and deallocation, allocate once at the
beginning and reuse in the iteration.

Signed-off-by: Timofey Titovets <nefelim4ag@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>