ext4: introduce EXT4_FC_TAG_BASE_LEN helper

Introduce EXT4_FC_TAG_BASE_LEN helper for calculate length of
struct ext4_fc_tl.

Signed-off-by: Ye Bin <yebin10@huawei.com>
Link: https://lore.kernel.org/r/20220924075233.2315259-2-yebin10@huawei.com


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