mkfs.xfs blank sw value mapped to (unsigned long)(-1), expensive when using swalloc

[Chris Pearson <kermit4@xxxxxxxxx>]

Blank sw mapped to -1 (must be an error value returned from some
string parsing function)

So it ends up 4294967264 if su is 128k, as that's equal to (unsigned
long)(-1 * su/bsize)

If you're mounting with swalloc, you'll quickly wind up with no free
space! Ouch!

# mkfs.xfs -V
mkfs.xfs version 3.1.4
# mkfs.xfs -dfile=1,name=hi,size=$((1024*1024*100)) -bsize=4k -d
su=128k,sw= -l version=2 -i size=2k -l lazy-count=1
meta-data=hi                     isize=2048   agcount=4, agsize=6400 blks
        =                       sectsz=512   attr=2, projid32bit=0
data     =                       bsize=4096   blocks=25600, imaxpct=25
        =                       sunit=32     swidth=4294967264 blks
naming   =version 2              bsize=4096   ascii-ci=0
log      =internal log           bsize=4096   blocks=1248, version=2
        =                       sectsz=512   sunit=32 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0