snapuserd: Move GetNumSectors call to snapuserd_server.

Calling this in snapuserd_core fails when the base path is a regular
file. Since the value is only read once, just call it from
snapuserd_server instead, which also means we don't have to add an
S_ISBLK check here.

Bug: 288273605
Test: snapuserd_test
Change-Id: Ic26bf807b24611f2d97829d1b4eb1d0ede2feb6a