fs: Fix uninitialized 'offp' in statmount_string()

In the Linux kernel, the following vulnerability has been resolved: fs: Fix uninitialized 'offp' in statmount_string() In statmount_string(), most flags assign an output offset pointer (offp) which is later updated with the string offset. However, the STATMOUNT_MNT_UIDMAP and STATMOUNT_MNT_GIDMAP cases directly set the struct fields instead of using offp. This leaves offp uninitialized, leading to a possible uninitialized dereference when *offp is updated. Fix it by assigning offp for UIDMAP and GIDMAP as well, keeping the code path consistent.