btrfs: send: limit number of clones and allocated memory size

The arg->clone_sources_count is u64 and can trigger a warning when a
huge value is passed from user space and a huge array is allocated.
Limit the allocated memory to 8MiB (can be increased if needed), which
in turn limits the number of clone sources to 8M / sizeof(struct
clone_root) = 8M / 40 = 209715.  Real world number of clones is from
tens to hundreds, so this is future proof.

Reported-by:  <syzbot+4376a9a073770c173269@syzkaller.appspotmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>