ext4: refactor out ext4_generic_attr_show()

Refactor out the function ext4_generic_attr_show() to handle the reading
of values of various common types, with no functional changes.

Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20240319113325.3110393-4-libaokun1@huawei.com


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