pstore: defer inserting OOPS entries into pstore

Life is simple for all the kernel terminating types of kmsg_dump
call backs - pstore just saves the tail end of the console log. But
for "oops" the situation is more complex - the kernel may carry on
running (possibly for ever).  So we'd like to make the logged copy
of the oops appear in the pstore filesystem - so that the user has
a handle to clear the entry from the persistent backing store (if
we don't, the store may fill with "oops" entries (that are also
safely stashed in /var/log/messages) leaving no space for real
errors.

Current code calls pstore_mkfile() immediately. But this may
not be safe. The oops could have happened with arbitrary locks
held, or in interrupt or NMI context. So allocating memory and
calling into generic filesystem code seems unwise.

This patch defers making the entry appear. At the time
of the oops, we merely set a flag "pstore_new_entry" noting that
a new entry has been added. A periodic timer checks once a minute
to see if the flag is set - if so, it schedules a work queue to
rescan the backing store and make all new entries appear in the
pstore filesystem.

Signed-off-by: Tony Luck <tony.luck@intel.com>

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