libext2fs: Round up the bitmap size when allocating a new bitmap

libext2fs: Round up the bitmap size when allocating a new bitmap

The x86 BT assembly instructure can overshoot the end of a bit array
when testing a bit at the end of the bit array, even if it never needs
to look at those memory locations.  This can cause a spurious
segmentation fault.  If we allocate a little extra memory, it avoids
this problem.  See:

	http://faydoc.tripod.com/cpu/bt.htm

This doesn't happen on Linux, probably because of the glibc's malloc()
function works, but apparently it's a major problem on the *BSD
operating systems.

Addresses-Sourceforge-Bug: #2328708

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