Calculate JFS size accurately
JFS was one of the file systems which was considered to have intrinsic unallocated space (reported size of the file system doesn't match the size of the partition). Commit 7ebedc4b "Don't show intrinsic unallocated space (#499202)". Both when using jfs-utils for unmounted and statvfs() when mounted.
Recent commit c15d0cd6 "Accept FS usage figures within significant unallocated threshold (!49 (merged))" peeked my interest. So I've read the mkfs.jfs source code and worked out how to accurately query and calculate the size of a JFS. This patchset is the result.