generic/397: be compatible with ignored SIGPIPE

generic/397: be compatible with ignored SIGPIPE

If generic/397 is executed in an environment with SIGPIPE ignored,
it fails because the 'yes' program prints an error message:

    yes: standard output: Broken pipe
    yes: write error

This can be reproduced with:

    trap '' SIGPIPE; ./check generic/397

Fix it by generating the string of 255 y's using just 'head' and
'tr' instead of 'yes', 'head', and 'tr'.

Although it's not really a good idea to execute xfstests with
SIGPIPE ignored, this is the only test I've noticed where it causes
a problem, so it might as well be fixed in the test.

It would be much nicer to prevent this problem for all tests by
making the 'check' script restore the default SIGPIPE handler.  But
that isn't straightforward because bash's 'trap' builtin doesn't
allow un-ignoring signals that were ignored on entry to the shell.

[ eguan added more background infomation to commit log, which is
  also from Eric.

I think it's an easy problem for others to run into, since sometimes
processes ignore SIGPIPE because they want to get write errors
instead, but then when doing fork() + exec() they forget to reset
the SIGPIPE handler. Notably, Python got this wrong and it wasn't
fixed until Python 3, so any programs executing the 'check' script
from a Python 2 script will usually get this wrong (see:
https://bugs.python.org/issue1652). And usually everything works
fine but every once in a while there is a weird problem like this
which has to be debugged. ]

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>