sctp: Fix iterator synchronization in sctp_sendall()

[markj]

Summary

- The SCTP_PCB_FLAGS_SND_ITERATOR_UP check was racy, since two threads
  could observe that the flag is not set and then both set it.  I'm not
  sure if this is actually a problem in practice, i.e., maybe there's no
  problem having multiple sends for a single PCB in the iterator list?
- sctp_sendall() was modifying sctp_flags without the inp lock held.

The change simply acquires the PCB write lock before toggling the flag,
fixing both problems.

N.B., sctp_sendall() seems buggy for other reasons: it appears to handle
uio == NULL (taking the input mbuf instead) but unconditionally
dereferences uio.