jfs: Switch to generic xattr handlers

This is mostly the same as on other filesystems except for attribute
names with an "os2." prefix: for those, the prefix is not stored on
disk, and on-attribute names without a prefix have "os2." added.

As on several other filesystems, the underlying function for
setting/removing xattrs (__jfs_setxattr) removes attributes when the
value is NULL, so the set xattr handlers will work as expected.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>