box: use xalloc in key_def_dump_parts

Temporary allocations from a region don't fail so let's use xalloc to
simplify the code.

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

(cherry picked from commit 28d51f8076b390c50d5b18f8e767c1eb540e5dcc)