[XFS] cleanup root inode handling in xfs_fs_fill_super

- rename rootvp to root for clarify
- remove useless vn_to_inode call
- check is_bad_inode before calling d_alloc_root
- use iput instead of VN_RELE in the error case

SGI-PV: 976035
SGI-Modid: xfs-linux-melb:xfs-kern:30708a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>