vfs: fix compat_sys_stat() handling of overflows in st_nlink

Massaged cp_compat_stat() into form closer to cp_new_stat(); the only
real issue had been in handling of st_nlink overflows - native 32bit
stat(2) returns -EOVERFLOW in such situations, compat one silently
loses upper bits.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>