Immutable Namespace Inode Cache for ZFS

With rev 14128 we now have support for ultra fast lookups in the immutable namespace hierarchy. A problem was spotted with zfs-fuse 0.6.9 where immns directory components were not being cached properly. This resulted in disk-bound lookup operations which drastically slowed performance of file creates. r14128 eliminates these lookups by prefetching the zfs inode numbers of immns directories prior to slashd startup.

(pauln@lemon:slash_nara)$ svn ci ../zfs/src/zfs-fuse/zfs_operations_slash.c slashd/main_mds.c
Sending        slash_nara/slashd/main_mds.c
Sending        zfs/src/zfs-fuse/zfs_operations_slash.c
Transmitting file data ..
Committed revision 14128.