RFR (XS) 8145593 - Clean up metaspaceShared.cpp

[Ioi Lam]

Please review a small fix:

http://cr.openjdk.java.net/~iklam/jdk9/8145593-metaspace-shared-cleanup.v01/

Bug: Clean up metaspaceShared.cpp

     https://bugs.openjdk.java.net/browse/JDK-8145593

Summary of fix:

    [1] The "md" and "mc" spaces are managed inside 
VM_PopulateDumpSharedSpace,
    which makes the implementation of 
MetaspaceShared::misc_data_space_alloc()
    very awkward. The relevant code should be moved into the 
MetaspaceShared class.

    [2] There is a block of code that appears twice in 
VM_PopulateDumpSharedSpace::doit().
    It should be cleaned up using a 2-iteration loop.

Tests:

     JPRT
     CDS tests (locally)

Thanks
- Ioi