ANDROID: GKI: fix ABI breakage in struct userfaultfd_ctx

The following two commits move 'userfaultfd_ctx' struct from
fs/userfaultfd.c to header file and then add a rw_semaphore to it. The
ABI is broken by the change. However, given that the type should be
private and not accessed by vendor modules, use some GENKSYMS #define
magic to preserve the CRC. Also update the .stg file for offset
adjustment within 'userfaultfd_ctx'.

5e4c24a5 ("userfaultfd: protect mmap_changing with rw_sem in userfaulfd_ctx")
f91e6b41 ("userfaultfd: move userfaultfd_ctx struct to header file")

Bug: 320478828
Change-Id: I5f97ff34dd8c88fe3d18c4dc902452488ba28cbd
Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>