[XFS] Kill off xfs_statvfs.

We were already filling the Linux struct statfs anyway, and doing this
trivial task directly in xfs_fs_statfs makes the code quite a bit cleaner.
While I was at it I also moved copying attributes that don't change over
the lifetime of the filesystem outside the superblock lock.

xfs_fs_fill_super used to get the magic number and blocksize through
xfs_statvfs, but assigning them directly is a lot cleaner and will save
some stack space during mount.

SGI-PV: 971186
SGI-Modid: xfs-linux-melb:xfs-kern:29802a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Tim Shimmin <tes@sgi.com>