Description of problem:
Anaconda when using --grow option for thinpool volume on RHEL 7.4 does not make calculation correctly, whereas RHEL 7.3 did 
Anaconda fails with traceback below when trying to create volume

Version-Release number of selected component (if applicable):
RHEL7.4
anaconda 21.48.22.121-1

How reproducible:
every time using attached kickstart file

Steps to Reproduce:
1. Created small vm with 40Gb space
2. install using attached kickstart
3. anaconda errors out

Actual results:
traceback

anaconda 21.48.22.121-1 exception report
Traceback (most recent call first):
  File "/usr/lib/python2.7/site-packages/blivet/devices/storage.py", line 488, in create
    raise errors.DeviceCreateError(str(e), self.name)
  File "/usr/lib/python2.7/site-packages/blivet/deviceaction.py", line 293, in execute
    self.device.create()
  File "/usr/lib/python2.7/site-packages/blivet/devicetree.py", line 373, in processActions
    action.execute(callbacks)
  File "/usr/lib/python2.7/site-packages/blivet/__init__.py", line 380, in doIt
    self.devicetree.processActions(callbacks)
  File "/usr/lib/python2.7/site-packages/blivet/__init__.py", line 230, in turnOnFilesystems
    storage.doIt(callbacks)
  File "/usr/lib64/python2.7/site-packages/pyanaconda/install.py", line 204, in doInstall
    turnOnFilesystems(storage, mountOnly=flags.flags.dirInstall, callbacks=callbacks_reg)
  File "/usr/lib64/python2.7/threading.py", line 765, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/usr/lib64/python2.7/site-packages/pyanaconda/threads.py", line 227, in run
    threading.Thread.run(self, *args, **kwargs)
DeviceCreateError: ('lvcreate failed for vg_system/lv_system_thinpool: running /sbin/lvm lvcreate --thinpool vg_system/lv_system_thinpool --size 9244m --poolmetadatasize 16 --chunksize 64 --config  devices { preferred_names=["^/dev/mapper
/", "^/dev/md/", "^/dev/sd"] }  failed', 'vg_system-lv_system_thinpool')


Expected results:
Installation to succeed

Additional info:
This disk layout worked on RHEL7.3

Further testing shows that specifying exact size works but using --grow doesn't
e.g.
logvol none --fstype="None" --thinpool --name=lv_system_thinpool --vgname=vg_system --chunksize=64 --size=9212 --metadatasize=16