feat(xfs): add Linux 7.0 XFS statistics fields

[dongjiang1989]

ref: https://elixir.bootlin.com/linux/v7.0/source/fs/xfs/xfs_stats.c

Summary

• Update XFS statistics parsing to match the Linux 7.0 kernel (fs/xfs/xfs_stats.c)
• Add support for 9 new btree stat groups: fibt2, rmapbt, refcntbt, rmapbt_mem, rcbagbt, rtrmapbt, rtrmapbt_mem, rtrefcntbt, rtrefcntbt — each with 15
  counters (lookup, compare, insrec, delrec, newroot, killroot, increment, decrement, lshift, rshift, split, join, alloc, free, moves)
• Add 2 new stat groups: zoned (gc_read_calls, gc_bytes) and metafile (inodes, meta)
• Add defer_relog (uint64) and gc xpc (uint64) extended precision counters
• Add trailing comments with proc label names to all Stats struct fields for easier reference
• Update test fixtures (proc and sys) to include all new fields
• Add unit tests for all new stat groups (happy path + invalid count cases)

Test plan

• go vet ./xfs/... passes
• go test ./xfs/... passes (TestParseStats, TestReadProcStat, TestReadSysStats)
• New inline tests cover bad/good cases for: fibt2, rmapbt, refcntbt, rmapbt_mem, rcbagbt, rtrmapbt, rtrmapbt_mem, rtrefcntbt, zoned, metafile, defer_relog, gc xpc

[dongjiang1989]

Hi @SuperQ , PTAL, thanks.

[dongjiang1989]

@discordianfish, @SuperQ Hi, I would appreciate your review of this PR. Thank you.