ext4: use saved local variable sbi instead of EXT4_SB(sb)

We save EXT4_SB(sb) to local variable sbi at beginning of function
ext4_resize_begin. Use sbi directly instead of EXT4_SB(sb) to
remove unnecessary pointer dereference.

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Link: https://lore.kernel.org/r/20230826174712.4059355-11-shikemeng@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>