pynfs: add visualization support for test results

Add 'make pynfs-visualize' target to generate comprehensive HTML reports
with PNG charts for pynfs test results. This makes it easy to understand
test outcomes at a glance and share results.

Features:
- HTML report with test summaries, statistics, and detailed results
- PNG charts showing test distribution (pie and bar charts)
- Comparison charts when multiple NFS versions are tested
- Automatic kernel version detection from results directory
- Self-contained output directory for easy transfer via scp

The visualization script generates:
- index.html: Main report with interactive tabs
- pynfs-v4_0-results.png: NFS v4.0 test charts
- pynfs-v4_1-results.png: NFS v4.1 test charts
- pynfs-vblock-results.png: pNFS block layout charts
- pynfs-comparison.png: Side-by-side version comparison

Usage:
  make pynfs-visualize                    # Auto-detect kernel
  make pynfs-visualize LAST_KERNEL=<version>  # Specific kernel

Output is generated in:
  workflows/pynfs/results/<kernel>/html/

Generated-by: Claude AI
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

Author: mcgrof