jaildesc: Publish the new fd only after the jaildesc is initialized

[markj]

Summary

jaildesc_alloc() finishes initializing the file structure only after it
is made visible from the file descriptor table via finit(). In that
window, other threads could try to perform operations on the descriptor
and thus access an incompletely initialized jaildesc.

Defer the finit() call until locks are initialized. While here,
simplify the error path for falloc_caps().