Remove libattr requirement
RHEL/CentOS 6 supports sys/xattr.h eliminating the need for
libattr-devel as a dependency.
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: DHE <git@dehacked.net>
Closes #7344
Closes #7351