fs/9p: convert mkdir to use get_new_inode

mkdir had different code paths for inode creation, cache used
the get_new_inode_from_fid helper, but non-cached used
v9fs_get_inode.  Collapsed into a single implementation across
both as there should be no difference.

Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>