initialize snap device for cow

[natalia-zelazna]

Hi, this is the complete work covering crash during reboot (handle_bdev_mount_event during transition from active to dormant state). This solution provides proper handling of cow_file_extens. It should solve #365

[Swistusmen]

What is the purpose of last commit? From what I see it in best case doesn't change anything because dentry and vfsmount are NULL when you do path_put so they just do return

[natalia-zelazna]

1. to remove direct dependency form version number in the code - I went throug older supported distros and dattobd works fine with kernel_path instead of user_path_at
2. to consistently (with other errors that may happen in this function) go to the error handling part without checking all other possible failures on the way.

[st-matskevich]

Closed as work will be completed in #375