xfs: create new metadata UUID field and incompat flag

This adds a new superblock field, sb_meta_uuid.  If set, along with
a new incompat flag, the code will use that field on a V5 filesystem
to compare to metadata UUIDs, which allows us to change the user-
visible UUID at will.  Userspace handles the setting and clearing
of the incompat flag as appropriate, as the UUID gets changed; i.e.
setting the user-visible UUID back to the original UUID (as stored in
the new field) will remove the incompatible feature flag.

If the incompat flag is not set, this copies the user-visible UUID into
into the meta_uuid slot in memory when the superblock is read from disk;
the meta_uuid field is not written back to disk in this case.

The remainder of this patch simply switches verifiers, initializers,
etc to use the new sb_meta_uuid field.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

diff --cc fs/xfs/libxfs/xfs_alloc.c
Simple merge

diff --cc fs/xfs/libxfs/xfs_alloc_btree.c
Simple merge

diff --cc fs/xfs/libxfs/xfs_attr_leaf.c
Simple merge

diff --cc fs/xfs/libxfs/xfs_attr_remote.c
Simple merge

diff --cc fs/xfs/libxfs/xfs_bmap_btree.c
Simple merge

diff --cc fs/xfs/libxfs/xfs_btree.c
Simple merge

diff --cc fs/xfs/libxfs/xfs_da_btree.c
Simple merge

diff --cc fs/xfs/libxfs/xfs_dir2_block.c
Simple merge

diff --cc fs/xfs/libxfs/xfs_dir2_data.c
Simple merge

diff --cc fs/xfs/libxfs/xfs_dir2_leaf.c
Simple merge

diff --cc fs/xfs/libxfs/xfs_dir2_node.c
Simple merge

diff --cc fs/xfs/libxfs/xfs_dquot_buf.c
Simple merge

diff --cc fs/xfs/libxfs/xfs_format.h
Simple merge

diff --cc fs/xfs/libxfs/xfs_ialloc.c
Simple merge

diff --cc fs/xfs/libxfs/xfs_ialloc_btree.c
Simple merge

diff --cc fs/xfs/libxfs/xfs_inode_buf.c
Simple merge

diff --cc fs/xfs/libxfs/xfs_sb.c
Simple merge

diff --cc fs/xfs/libxfs/xfs_symlink_remote.c
Simple merge