MFC r313910: Change XCOPY memory allocations.

Before this change XCOPY code could allocate memory in chunks up to 16-32MB
(VMware does XCOPY in 4MB chunks by default), that could be difficult for
VM subsystem to do due to KVA fragmentation, that sometimes created huge
allocation delays, blocking any I/O for respective LU for that time.

This change limits allocations down to TPC_MAX_IO_SIZE, which is 1MB now.
1MB is also not a cookie, but ZFS also can do that for large blocks, so
it should be less dramatic. As drawback this increases CPU overhead, but
it still look acceptable comparing to time consumed by ZFS read/write.