libperf: Add preadn()

Add preadn() to provide pread() and readn() semantics.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/ab8918a4-7ac8-a37e-2e2c-28438c422d87@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>