ext4: factor out ext4_fc_get_tl()

Factor out ext4_fc_get_tl() to fill 'tl' with host byte order.

Signed-off-by: Ye Bin <yebin10@huawei.com>
Link: https://lore.kernel.org/r/20220924075233.2315259-3-yebin10@huawei.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>