GFS2: Use proper allocation context for new inodes

Rather than using the parent directory's allocation context, this
patch allocated the new inode earlier in the process and then uses
it to contain all the information required. As a result, we can now
use the new inode's own allocation context to allocate it rather
than having to use the parent directory's context. This give us a
lot more flexibility in where the inode is placed on disk.
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>