kernel - Adjust ssb_space_prealloc() use cases
* Add two flags to the signalsockbuf ssb_flags field.
SSB_PREALLOC - Indicates that data preallocation tracking is being used
SSB_STOPSUPP - Indicates that SSB_STOP flow control is being used
* unix domain sockets set SSB_STOPSUPP, tcp and sctp sockets
set SSB_PREALLOC.
* sendfile() requires that either SSB_PREALLOC or SSB_STOPSUPP be specified.
* Code now conditionalizes the use of ssb_space() vs ssb_space_prealloc()
based on the presence of the SSB_PREALLOC flag.
Reported-by: sephe