Refactor extents calculations / updates in _lvcreate.

Refactor extents calculations / updates in _lvcreate.

Move extents calculation adjustments into their own local functions
right after we read the vg.  This calculation really is not part of
the LV create function but is rather an adjustment to the parameters
based on what is given on the cmdline.  So we move it outside the main
_lvcreate.

Should be no functional change.

Author: Dave Wysochanski <dwysocha@redhat.com>