generic/020: fix max_attrval_size for XFS, UDF, Btrfs and NFS

generic/020: fix max_attrval_size for XFS, UDF, Btrfs and NFS

As found by Dave Chinner, fff4359d ("020: make this xattr test generic")
unintentionally changed the long attribute value length from 100K to 64
*bytes* for XFS, UDF and Btrfs.
Update XFS and UDF to use a 64K $max_attrval_size value. For Btrfs, use
the nodesize, xattr length and tree entry overhead sizes to calculate
the maximum.
NFS doesn't provide a way to find out the $max_attrval_size for the
underlying filesystem on the server, so just use a rough 1K limit.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>