ext4: fold ext4_data_block_valid_rcu() into the caller

After the previous patch, ext4_data_block_valid_rcu() has a single
caller. Fold it into it.

Reviewed-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20200728130437.7804-5-jack@suse.cz


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