[FIX] buffer: kernel crash caused by splice_read

[FIX] buffer: kernel crash caused by splice_read

 - splice_grow_spd must be done _before_ spd structure filling;
 - number of pages and partial must be PIPE_DEF_BUFFERS;
 - pages that will be passed to splice_to_pipe must be copied, not
   moved: it is unknown when they will be returned by tcp stack, so we
   must not rely on their reuse;
 - small refactoring.

Known issue:
 - If a subbuffer is greater than max pipe size, we should splice this
   subbuffer partially. So we should not put it to "write list". In
   general a new mechanism for this should be implemented.

Tested with:
 - 2 x 8Kb buffer;
 - 16 x 64Kb buffer.