tracing: Fix comment in tracing_buffers_splice_read()

tracing: Fix comment in tracing_buffers_splice_read()

The comment about returning an error if the read fails on the first
iteration is slightly incorrect. It makes it sound like the only reason it
could fail on a later iteration is if the subbuf order changed. That is
incorrect, it could also fail if the length passed in was not a multiple
of the subbuf size. Fix the comment.

Link: https://lore.kernel.org/all/20260904143527.40e73d36@gandalf.local.home/
Link: https://patch.msgid.link/20260904144902.506862a1@gandalf.local.home
Fixes: dae8dda341d2 ("tracing: Fix subbuf resize races with trace_pipe_raw readers")
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>