btrfs: simplify internal btrfs_printk helpers

btrfs: simplify internal btrfs_printk helpers

The printk() can be compiled out depending on CONFIG_PRINTK, this is
reflected in our helpers. The indirection is provided by btrfs_printk()
used in the ratelimited and RCU wrapper macros.

Drop the btrfs_printk() helper and define the ratelimit and RCU helpers
directly when CONFIG_PRINTK is undefined. This will allow further
changes to the _btrfs_printk() interface (which is internal), any
message in other code should use the level-specific helpers.

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