[IA64] resolve name clash by renaming is_available_memory()

There is a name clash with ia64 arch code in Andrew's tree. Rename
is_avialable_memory() to is_memory_available() to avoid the clash.

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>