mm: initialize return of vm_insert_pages

mm: initialize return of vm_insert_pages

clang static analysis reports a garbage return

  In file included from mm/memory.c:84:
  mm/memory.c:1612:2: warning: Undefined or garbage value returned to caller [core.uninitialized.UndefReturn]
          return err;
          ^~~~~~~~~~

The setting of err depends on a loop executing.  So initialize err.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20200703155354.29132-1-trix@redhat.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>