missing bits of "splice: fix racy pipe->buffers uses"

that commit has fixed only the parts of that mess in fs/splice.c itself;
there had been more in several other ->splice_read() instances...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --cc kernel/relay.c
Simple merge

diff --cc kernel/trace/trace.c
Simple merge

diff --cc mm/shmem.c
Simple merge