[Feature request] flag for `btrfs check start` to redirect messages from dmesg to stdout

[solomoncyj]

Prevent spamming of dmesg , which should be should be reserved for kernel messages or messages with a serverity of notice or higer. this also prevents the need of the user having to open a 2nd terminal window to watch dmesg to check for progress

[solomoncyj]

flooding dmesg with messages like this may pevent the user from being able to find important information