xfs_scrub: require primary superblock repairs to complete before proceeding

xfs_scrub: require primary superblock repairs to complete before proceeding

Phase 2 of the xfs_scrub program calls the kernel to check the primary
superblock before scanning the rest of the filesystem.  Though doing so
is a no-op now (since the primary super must pass all checks as a
prerequisite for mounting), the goal of this code is to enable future
kernel code to intercept an xfs_scrub run before it actually does
anything.  If this some day involves fixing the primary superblock, it
seems reasonable to require that /all/ repairs complete successfully
before moving on to the rest of the filesystem.

Unfortunately, that's not what xfs_scrub does now -- primary super
repairs that fail are theoretically deferred to phase 4!  So make this
mandatory.

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