xfs: allow single bulkstat of special inodes

Create a new bulk ireq flag that enables userspace to ask us for a
special inode number instead of interpreting @ino as a literal inode
number.  This enables us to query the root inode easily.

The reason for adding the ability to query specifically the root
directory inode is that certain programs (xfsdump and xfsrestore) want
to confirm when they've been pointed to the root directory.  The
userspace code assumes the root directory is always the first result
from calling bulkstat with lastino == 0, but this isn't true if the
(initial btree roots + initial AGFL + inode alignment padding) is itself
long enough to be allocated to new inodes if all of those blocks should
happen to be free at the same time.  Rather than make userspace guess
at internal filesystem state, we provide a direct query.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>