btrfs: pass level to _btrfs_printk() to avoid parsing level from string

btrfs: pass level to _btrfs_printk() to avoid parsing level from string

There's code in _btrfs_printk() to parse the message level from the
input string so we can augment the message with the level description
for better visibility in the logs.

The parsing code has evolved over time, see commits:

- 40f7828b36e3b9 ("btrfs: better handle btrfs_printk() defaults")

- 262c5e86fec7cf ("printk/btrfs: handle more message headers")

- 533574c6bc30cf ("btrfs: use printk_get_level and printk_skip_level, add __printf, fix fallout")

- 4da35113426d16 ("btrfs: add varargs to btrfs_error")

As we are using the specific level helpers everywhere we can simply pass
the message level so we don't have to parse it. The proper printk()
message header is created as KERN_SOH + "level".

Signed-off-by: David Sterba <dsterba@suse.com>