nfs/localio: add tracepoints for misaligned DIO READ and WRITE support

Add nfs_local_dio_class and use it to create nfs_local_dio_read,
nfs_local_dio_write and nfs_local_dio_misaligned trace events.

These trace events show how NFS LOCALIO splits a given misaligned
IO into a mix of misaligned head and/or tail extents and a DIO-aligned
middle extent.  The misaligned head and/or tail extents are issued
using buffered IO and the DIO-aligned middle is issued using O_DIRECT.

This combination of trace events is useful for LOCALIO DIO READs:

  echo 1 > /sys/kernel/tracing/events/nfs/nfs_local_dio_read/enable
  echo 1 > /sys/kernel/tracing/events/nfs/nfs_local_dio_misaligned/enable
  echo 1 > /sys/kernel/tracing/events/nfs/nfs_initiate_read/enable
  echo 1 > /sys/kernel/tracing/events/nfs/nfs_readpage_done/enable
  echo 1 > /sys/kernel/tracing/events/xfs/xfs_file_direct_read/enable

This combination of trace events is useful for LOCALIO DIO WRITEs:

  echo 1 > /sys/kernel/tracing/events/nfs/nfs_local_dio_write/enable
  echo 1 > /sys/kernel/tracing/events/nfs/nfs_local_dio_misaligned/enable
  echo 1 > /sys/kernel/tracing/events/nfs/nfs_initiate_write/enable
  echo 1 > /sys/kernel/tracing/events/nfs/nfs_writeback_done/enable
  echo 1 > /sys/kernel/tracing/events/xfs/xfs_file_direct_write/enable

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>

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