Btrfs-progs: make pretty_sizes take u64 instead of a double

Btrfs-progs: make pretty_sizes take u64 instead of a double

This got changed to a double but all the callers still use a u64, which causes
us to segfault sometimes because of some weird C voodoo that I had to have
explained to me.  Apparently because we're using a double the compiler will use
the floating point registers to hold our argument which ends up not being
aligned properly if you don't actually give it a double so it will cause
problems for other things, in our case it was screwing up str_bytes so it was
larger than the actual size of the str.  This patch fixes the segfault.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>