MFp4: Simplify code a bit:

• Remove kern.geom.mirror.sync_block_size sysctl. It is quite obvious that we want to use the biggest size possible.
• Do not use UMA zone for sync data allocations. There could be only one synchronization request per synchronized disk at a time, so allocate memory for one request on whole synchronization process related to one disk.

Tested by synchronizing one component (out of three) and by synchronizing
two components (out of three) in parallel.