st: implement tape statistics

This patch implements tape statistics in the st module via
sysfs. Current no statistics are available for tape I/O and there
is no easy way to reuse the block layer statistics for tape
as tape is a character device and does not have perform I/O in
sector sized chunks (the size of the data written to tape
can change). For tapes we also need extra stats related to
things like tape movement (via other I/O).

There have been multiple end users requesting statistics
including AT&T (and some HP customers who have not given
permission to be named). It is impossible for them
to investigate any issues related to tape performance
in a non-invasive way.

[jejb: eliminate PRId64]
Signed-off-by: Shane Seymour <shane.seymour@hp.com>
Tested-by: Shane Seymour <shane.seymour@hp.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: James Bottomley <JBottomley@Odin.com>

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