fs: fix hang with BSD accounting on frozen filesystem

fs: fix hang with BSD accounting on frozen filesystem

When BSD process accounting is enabled and logs information to a
filesystem which gets frozen, system easily becomes unusable because each
attempt to account process information blocks.  Thus e.g.  every task gets
blocked in exit.

It seems better to drop accounting information (which can already happen
when filesystem is running out of space) instead of locking system up.  So
we open the accounting file with O_NONBLOCK.

Signed-off-by: Jan Kara <jack@suse.cz>
Reported-by: Nikola Ciprich <nikola.ciprich@linuxbox.cz>
Tested-by: Nikola Ciprich <nikola.ciprich@linuxbox.cz>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Marco Stornelli <marco.stornelli@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>