jbd2: remove unused exports for jbd2 debugging

Jbd2 exports jbd2_journal_enable_debug and __jbd2_debug() depite the
first is used only in fs/jbd2/journal.c and the second only within jbd2
code. Remove the pointless exports make jbd2_journal_enable_debug
static.

Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Lukas Czerner <lczerner@redhat.com>
Link: https://lore.kernel.org/r/20220608112355.4397-3-jack@suse.cz


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