sysfs: add InfiniBand device listing and per-device parsing

[rafal-szypulka]

Summary

Add APIs that allow callers to enumerate InfiniBand device names without
reading device attributes or counters, and then parse selected devices
individually:

• FS.InfiniBandClassDevices() lists device names under
  /sys/class/infiniband without parsing them.
• FS.InfiniBandDevice(name) parses one selected device.
• FS.InfiniBandClass() is refactored to compose these APIs while preserving
  its existing behavior.

Motivation

InfiniBandClass() currently parses every device eagerly. This prevents
callers such as node_exporter from applying device filters before sysfs
counter reads occur.

On NVIDIA systems with Fabric Manager-managed or otherwise restricted
Mellanox PFs, merely reading some counters can trigger repeated
ACCESS_REG(0x805) firmware errors. Filtering the returned
InfiniBandClass is too late because the reads have already happened.

Separating enumeration from parsing lets callers:

1. enumerate device names;
2. apply their own include/exclude policy;
3. parse only accepted devices.

The existing InfiniBandClass() API and default behavior remain unchanged.

Tests

• Verify that device names can be enumerated even when the device directories
  do not contain readable attributes or counters.
• Verify that InfiniBandDevice(name) returns the same data as the
  corresponding InfiniBandClass() entry.
• Linux test package cross-compiles successfully.

Related to #823 and prometheus/node_exporter#3434.

Existing work

#839 already proposes the additive InfiniBandDevice(name) API. This change
includes that capability and additionally provides list-only enumeration so
callers do not need to reproduce procfs sysfs path handling outside this
package.