sctp: Fix iterator synchronization in sctp_sendall()

• 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.

Reviewed by: tuexen
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 173a7a4ee4fa8fbce6b4532d4d324bc72ee25fe0)