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

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.

Reviewed-by: Martijn Coenen <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
CC: [email protected] # Prerequisite for "writeback: Avoid skipping inode writeback"
Signed-off-by: Jan Kara <[email protected]>