perf bench inject-buildid: Handle writen() errors

perf bench inject-buildid: Handle writen() errors

The build on fedora:35 and fedora:rawhide with clang is failing with:

  49    41.00 fedora:35                     : FAIL clang version 13.0.0 (Fedora 13.0.0~rc1-1.fc35)
    bench/inject-buildid.c:351:6: error: variable 'len' set but not used [-Werror,-Wunused-but-set-variable]
            u64 len = 0;
                ^
    1 error generated.
    make[3]: *** [/git/perf-5.14.0-rc7/tools/build/Makefile.build:139: bench] Error 2
  50    41.11 fedora:rawhide                : FAIL clang version 13.0.0 (Fedora 13.0.0~rc1-1.fc35)
    bench/inject-buildid.c:351:6: error: variable 'len' set but not used [-Werror,-Wunused-but-set-variable]
            u64 len = 0;
                ^
    1 error generated.
    make[3]: *** [/git/perf-5.14.0-rc7/tools/build/Makefile.build:139: bench] Error 2

That 'len' variable is not used at all, so just make sure all the
synthesize_RECORD() routines return ssize_t to propagate the writen()
return, as it may fail, ditch the 'ret' var and bail out if those
routines fail.

Fixes: 0bf02a0d80427f26 ("perf bench: Add build-id injection benchmark")
Acked-by: Namhyung Kim <namhyung@kernel.org>
Link: http://lore.kernel.org/lkml/CAM9d7cgEZNSor+B+7Y2C+QYGme_v5aH0Zn0RLfxoQ+Fy83EHrg@mail.gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>