BUG/MINOR: threads: don't drop "extern" on the lock in include files

Commit 9dcf9b6 ("MINOR: threads: Use __decl_hathreads to declare locks")
accidently lost a few "extern" in certain lock declarations, possibly
causing certain entries to be declared at multiple places. Apparently
it hasn't caused any harm though.

The offending ones were :
  - fdtab_lock
  - fdcache_lock
  - poll_lock
  - buffer_wq_lock
2 files changed