ext4: show the default enabled prefetch_block_bitmaps option

ext4: show the default enabled prefetch_block_bitmaps option

After commit 21175ca434c5 ("ext4: make prefetch_block_bitmaps default"),
we enable 'prefetch_block_bitmaps' by default, but this is not shown in
the '/proc/fs/ext4/sdx/options' procfs interface.

This makes it impossible to distinguish whether the feature is enabled by
default or not, so 'prefetch_block_bitmaps' is shown in the 'options'
procfs interface when prefetch_block_bitmaps is enabled by default.

This makes it easy to notice changes to the default mount options between
versions through the '/proc/fs/ext4/sdx/options' procfs interface.

Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20241008120134.3758097-1-libaokun@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>