btrfs: turn name_cache radix tree into XArray in send_ctx

… and adjust all usages of this object to use the XArray API for the sake
of consistency.

XArray API provides array semantics, so it is notionally easier to use and
understand, and it also takes care of locking for us.

None of this makes a real difference in this particular patch, but it does
in other places where similar replacements are or have been made and we
want to be consistent in our usage of data structures in btrfs.

Signed-off-by: Gabriel Niebler <gniebler@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>