jbd2: unexport jbd2_log_start_commit()

jbd2_log_start_commit() is not used outside of jbd2 so unexport it. Also
make __jbd2_log_start_commit() static when we are at it.

Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Lukas Czerner <lczerner@redhat.com>
Link: https://lore.kernel.org/r/20220608112355.4397-4-jack@suse.cz


Signed-off-by: Theodore Ts'o <tytso@mit.edu>