unionfs: rework pathname handling

Running stress2 unionfs tests reliably produces a namei_zone corruption
panic due to unionfs_relookup() attempting to NUL-terminate a newly-
allocate pathname buffer without first validating the buffer length.

Instead, avoid allocating new pathname buffers in unionfs entirely,
using already-provided buffers while ensuring the the correct flags
are set in struct componentname to prevent freeing or manipulation
of those buffers at lower layers.

While here, also compute and store the path length once in the unionfs
node instead of constantly invoking strlen() on it.

Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D31728