xfs: add an xfs_zero_eof() tracepoint

Add a tracepoint in xfs_zero_eof() to facilitate tracking and debugging
EOF zeroing events. This has proven useful in the context of other
direct I/O tracepoints to ensure EOF zeroing occurs within appropriate
file ranges.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>