ext4: factor out ext4_fc_disabled()

Factor out ext4_fc_disabled(). No functional change.

Signed-off-by: Ye Bin <yebin10@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220916083836.388347-2-yebin10@huawei.com


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