nilfs2: do not allocate nilfs_mdt_info structure to gc-inodes

GC-inode now doesn't need the nilfs_mdt_info structure and there is no
reason that it is a sort of metadata files.

This stops the allocation and makes them not dependent on metadata
file routines.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>