f2fs: fix to add missing iput() in gc_data_segment()

During gc_data_segment(), if inode state is abnormal, it missed to call
iput(), fix it.

Fixes: b73e5282 ("f2fs: reposition unlock_new_inode to prevent accessing invalid inode")
Fixes: 9056d648

 ("f2fs: fix to do sanity check on inode type during garbage collection")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>