erofs: kill erofs_{init,exit}_inode_cache

As Christoph said [1] "having this function seems
entirely pointless", let's kill those.

filesystem                              function name
ext2,f2fs,ext4,isofs,squashfs,cifs,...  init_inodecache

In addition, add a necessary "rcu_barrier()" on exit_fs();

[1] https://lore.kernel.org/r/20190829101545.GC20598@infradead.org/


Reported-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190904020912.63925-9-gaoxiang25@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>