[XFS] cleanup xfs_vn_mknod

- use proper goto based unwinding instead of the current mess of
  multiple conditionals
- rename ip to inode because that's the normal convention for Linux
  inodes while ip is the convention for xfs_inodes
- remove unlikely checks for the default_acl - branches marked unlikely
  might lead to extreme branch bredictor slowdons if taken and for some
  workloads a default acl is quite common
- properly indent the switch statements
- remove xfs_has_fs_struct as nfsd has a fs_struct in any semi-recent
  kernel

SGI-PV: 976035
SGI-Modid: xfs-linux-melb:xfs-kern:30529a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>