Allow FreeBSD mount NetBSD's FFSv2 WAPBL read-only

[ricardobranco777]

ufs: Allow read-only mounting of NetBSD FFSv2 WAPBL filesystems

Skip UFS2 fs_metaspace upper-bound validation that rejects NetBSD FFSv2
WAPBL filesystems due to differing superblock layouts.

Detect the condition during mount instead and permit read-only mounts
while rejecting read-write mounts with EROFS.  This follows NetBSD's
recommendation for systems without WAPBL support and avoids modifying
unsupported journal metadata.

Fixes https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=296022

[asomers]

This patch may be sufficient to mount, but what guarantees are there that it will work correctly? That all types of metadata will be readable, and that performing writes on FreeBSD won't render the volume corrupt from NetBSD's perspective? It will take a very detailed understanding of both operating systems' on-disk formats to answer those questions.
cc @realchonk @mckusick .

[ricardobranco777]

This patch may be sufficient to mount, but what guarantees are there that it will work correctly? That all types of metadata will be readable, and that performing writes on FreeBSD won't render the volume corrupt from NetBSD's perspective? It will take a very detailed understanding of both operating systems' on-disk formats to answer those questions. cc @realchonk @mckusick .

fs_unrefs is used only once in sys/ufs/ufs/ufs_gjournal.c. Ditto for fs_providersize in sys/geom/label/g_label_ufs.c. These code paths are touched only if the user runs gjournal label on the device.

fs_metaspace seems safe for read-only mounts but I'm not so sure for read-write.

I'd be honoured to have a review by Marshall Kirk McKusick. :)