xfs: check if an open file is on the health monitored fs

xfs: check if an open file is on the health monitored fs

Create a new ioctl for the healthmon file that checks that a given fd
points to the same filesystem that the healthmon file is monitoring.
This allows xfs_healer to check that when it reopens a mountpoint to
perform repairs, the file that it gets matches the filesystem that
generated the corruption report.

(Note that xfs_healer doesn't maintain an open fd to a filesystem that
it's monitoring so that it doesn't pin the mount.)

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>