xfs: remove m_attroffset

With the upcoming v3 inodes the default attroffset needs to be calculated
for each specific inode, so we can't cache it in the superblock anymore.

Also replace the assert for wrong inode sizes with a proper error check
also included in non-debug builds.  Note that the ENOSYS return for
that might seem odd, but that error is returned by xfs_mount_validate_sb
for all theoretically valid but not supported filesystem geometries.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Josef 'Jeff' Sipek <jeffpc@josefsipek.net>