btrfs: tracepoints: convert flush states to using EM macros
Only 6 out of all flush states were being printed correctly since
only they were exported via the TRACE_DEFINE_ENUM macro. This patch
converts all flush states to use the newly introduced EM macro so that
they can all be printed correctly.
Signed-off-by: Nikolay Borisov <nborisov@suse.com> Reviewed-by: David Sterba <dsterba@suse.com> Signed-off-by: David Sterba <dsterba@suse.com>