fib_dxr: set fib_data field in struct dxr_aux early enough

Previously it was possible for dxr_build() to return with da->fd
unset in case of range_tbl or x_tbl malloc() failures. This
may have led to NULL ptr dereferencing in dxr_change_rib_batch().

Approved by: re (cperciva)
MFC after: 1 week

PR: 278422
(cherry picked from commit 0418d7a0903725ade71ae77c4ff900010a93a185)