ext4: convert EXT4_B2C(sbi->s_stripe) users to EXT4_NUM_B2C

ext4: convert EXT4_B2C(sbi->s_stripe) users to EXT4_NUM_B2C

Although we have checks to make sure s_stripe is a multiple of cluster
size, in case we accidentally end up with a scenario where this is not
the case, use EXT4_NUM_B2C() so that we don't end up with unexpected
cases where EXT4_B2C(stripe) becomes 0.

Also make the is_stripe_aligned check in regular_allocator a bit more
robust while we are at it. This should ideally have no functional change
unless we have a bug somewhere causing (stripe % cluster_size != 0)

Reviewed-by: Kemeng Shi <shikemeng@huaweicloud.com>
Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Link: https://patch.msgid.link/e0c0a3b58a40935a1361f668851d041575861411.1725002410.git.ojaswin@linux.ibm.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>