writeback: Protect inode->i_io_list with inode->i_lock

Currently, operations on inode->i_io_list are protected by
wb->list_lock. In the following patches we'll need to maintain
consistency between inode->i_state and inode->i_io_list so change the
code so that inode->i_lock protects also all inode's i_io_list handling.

Change-Id: Idad8e2028465703d5e48fd22b8a568eb39530864
Reviewed-by: Martijn Coenen <maco@android.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
CC: stable@vger.kernel.org # Prerequisite for "writeback: Avoid skipping inode writeback"
Signed-off-by: Jan Kara <jack@suse.cz>
Git-commit: e11ead2b
Git-repo: https://android.googlesource.com/kernel/common/


Signed-off-by: Pradeep P V K <ppvk@codeaurora.org>