ring-buffer: Have 32 bit time stamps use all 64 bits

When the new logic was made to handle deltas of events from interrupts
that interrupted other events, it required 64 bit local atomics.
Unfortunately, 64 bit local atomics are expensive on 32 bit architectures.
Thus, commit 10464b4aa605e ("ring-buffer: Add rb_time_t 64 bit operations
for speeding up 32 bit") created a type of seq lock timer for 32 bits.
It used two 32 bit local atomics, but required 2 bits from them each for
synchronization, making it only 60 bits.

Add a new "msb" field to hold the extra 4 bits that are cut off.

Link: https://lore.kernel.org/all/20220426175338.3807ca4f@gandalf.local.home/
Link: https://lkml.kernel.org/r/20220427170812.53cc7139@gandalf.local.home

Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

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