i915: fix missing user_access_end() in page fault exception case

When commit fddcd00a49 ("drm/i915: Force the slow path after a
user-write error") unified the error handling for various user access
problems, it didn't do the user_access_end() that is needed for the
unsafe_put_user() case.

It's not a huge deal: a missed user_access_end() will only mean that
SMAP protection isn't active afterwards, and for the error case we'll be
returning to user mode soon enough anyway.  But it's wrong, and adding
the proper user_access_end() is trivial enough (and doing it for the
other error cases where it isn't needed doesn't hurt).

I noticed it while doing the same prep-work for changing
user_access_begin() that precipitated the access_ok() changes in commit
96d4f267e4 ("Remove 'type' argument from access_ok() function").

Fixes: fddcd00a49 ("drm/i915: Force the slow path after a user-write error")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: stable@kernel.org # v4.20
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -1605,6 +1605,7 @@ static int eb_copy_relocations(const struct i915_execbuffer *eb)
 					     (char __user *)urelocs + copied,
 					     len)) {
 end_user:
+				user_access_end();
 				kvfree(relocs);
 				err = -EFAULT;
 				goto err;