ia64: add __NR_umount2 definition

Other architectures commonly use __NR_umount2 for sys_umount,
only ia64 and alpha use __NR_umount here. In order to synchronize
the generated tables, use umount2 like everyone else, and add back
the old name from asm/unistd.h for compatibility.

The __IGNORE_* lines are now all obsolete and can be removed as
a side-effect.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>