nilfs2: fix regression of garbage collection ioctl

On 2.6.37-rc1, garbage collection ioctl of nilfs was broken due to the
commit 263d90ce ("nilfs2: remove own inode hash used for GC"),
and leading to filesystem corruption.

The patch doesn't queue gc-inodes for log writer if they are reused
through the vfs inode cache.  Here, gc-inode is the inode which
buffers blocks to be relocated on GC.  That patch queues gc-inodes in
nilfs_init_gcinode() function, but this function is not called when
they don't have I_NEW flag.  Thus, some of live blocks are wrongly
overrode without being moved to new logs.

This resolves the problem by moving the gc-inode queueing to an outer
function to ensure it's done right.

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