ocfs2: split out inode alloc code from ocfs2_mknod_locked

Do this by splitting the bulk of the function away from the inode allocation
code at the very tom of ocfs2_mknod_locked(). Existing callers don't need to
change and won't see any difference. The new function created,
__ocfs2_mknod_locked() will be used shortly.

Signed-off-by: Mark Fasheh <mfasheh@suse.com>
Signed-off-by: Tao Ma <tao.ma@oracle.com>