NFS: Use nfs_fs_mount_common() for xdev mounts

At this point, there are only a few small differences between these two
functions.  I can set a few function pointers in the nfs_mount_info
struct to get around these differences.

Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>