libext2fs: fix potential integer overflow in bitmap accessors

libext2fs: fix potential integer overflow in bitmap accessors

bmap->cluster_bits has a maximum value of 19, but Coverity doesn't
know that.  To make it happy, and just in case there is a bug where
somehow the cluster size does get set to an invalid value and the rest
of the library doesn't check it, use 1ULL instead of 1 to avoid the
integer overflow.

Addresses-Coverity-Bug: 1500759
Addresses-Coverity-Bug: 1500764
Addresses-Coverity-Bug: 1500771
Signed-off-by: Theodore Ts'o <tytso@mit.edu>