mkfs.xfs: Don't stagger AG for a single disk

mkfs.xfs: Don't stagger AG for a single disk

When sunit and swidth are used mkfs.xfs tries to avoid all allocation
groups aligning on the same stripe and will attempt to stagger them
across the stripes that make up swidth.  If there is only one stripe
then there is no benefit in this optimisation.

$ truncate -s10G xfs_10G_su256k_sw1.image
$ mkfs.xfs -d su=256k,sw=1 xfs_10G_su256k_sw1.image
meta-data=xfs_10G_su256k_sw1.image isize=512    agcount=16, agsize=163776 blks
         =                       sectsz=512   attr=2, projid32bit=1
         =                       crc=1        finobt=0, sparse=0
data     =                       bsize=4096   blocks=2620416, imaxpct=25
         =                       sunit=64     swidth=64 blks
naming   =version 2              bsize=4096   ascii-ci=0 ftype=1
log      =internal log           bsize=4096   blocks=2560, version=2
         =                       sectsz=512   sunit=64 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0

A side effect of the optimisation is that the size adjustment used to stager
the allocation groups causes the last sunit of storage to be unused.

$ echo $((2620416*4096))
10733223936
$ ls -l xfs_10G_su256k_sw1.image
-rw-rw-r--. 1 test test 10737418240 Aug 30 10:54 xfs_10G_su256k_sw1.image

Skip this optimisation when sunit == swidth.

Signed-off-by: Donald Douwsma <ddouwsma@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>