xfs: don't commit sunit/swidth updates to disk if that would cause repair failures
Alex Lyakas reported[1] that mounting an xfs filesystem with new sunit and swidth values could cause xfs_repair to fail loudly. The problem here is that repair calculates the where mkfs should have allocated the root inode, based on the superblock geometry. The allocation decisions depend on sunit, which means that we really can't go updating sunit if it would lead to a subsequent repair failure on an otherwise correct filesystem. Port from xfs_repair some code that computes the location of the root inode and teach mount to skip the ondisk update if it would cause problems for repair. Along the way we'll update the documentation, provide a function for computing the minimum AGFL size instead of open-coding it, and cut down some indenting in the mount code. Note that we allow the mount to proceed (and new allocations will reflect this new geometry) because we've never screened this kind of thing before. We'll have to wait for a new future incompat feature to enforce correct behavior, alas. Note that the geometry reporting always uses the superblock values, not the incore ones, so that is what xfs_info and xfs_growfs will report. [1] https://lore.kernel.org/linux-xfs/20191125130744.GA44777@bfoster/T/#m00f9594b511e076e2fcdd489d78bc30216d72a7d Reported-by:Alex Lyakas <alex@zadara.com> Signed-off-by: