ext4: fsmap: fix the block/inode bitmap comment

While debugging fstest ext4/027 failure, found below comment to be wrong and
confusing. Hence fix it while we are at it.

Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/e79134132db7ea42f15747b5c669ee91cc1aacdf.1622432690.git.riteshh@linux.ibm.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>