staging: exfat: properly support discard in clr_alloc_bitmap()

Currently the discard code in clr_alloc_bitmap() is just dead code.
Move code around so that the discard operation is properly attempted
when enabled.

Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Link: https://lore.kernel.org/r/20191205152913.GJ3276@xps-13


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>