XFS inode reclaim stats

[bobrik]

I would like to see XFS inode reclaim metrics:

• https://github.com/prometheus/procfs/blob/619930b0b47/xfs/xfs.go#L95

From http://xfs.org/index.php/Runtime_Stats:

This is the number of times the operating system recycled an XFS inode from the inode cache in order to use the memory for that inode for another purpose. Inodes are recycled in order to keep the inode cache from growing without bound. If the reclaim rate is high it may be beneficial to raise the vnode_free_ratio kernel tunable variable to increase the size of the inode cache.

I'm happy to make a PR, but worry that it will stall just like prometheus/procfs#106 did.