lvconvert: limit libdm to maximum of 64 RAID devices

lvconvert: limit libdm to maximum of 64 RAID devices

Commit 64a2fad5d6c6 raised the maximum number of RAID devices to 64.

Commit e2354ea344c2 introduced RAID_BITMAP_SIZE as 4 to have
256 bits (4 * 64 bit array members), thus changing the libdm API
unnecessarilly for the time being.

To not change the API, reduce RAID_BITMAP_SIZE to 1.
Remove an unneeded definition of it from libdm-common.h.

If we ever decide to raise past 64, we'll version the API.

Related: rhbz834579
Related: rhbz1191935
Related: rhbz1191978