usercopy: Handle vm_map_ram() areas

vmalloc does not allocate a vm_struct for vm_map_ram() areas.  That causes
us to deny usercopies from those areas.  This affects XFS which uses
vm_map_ram() for its directories.

Fix this by calling find_vmap_area() instead of find_vm_area().

Fixes: 0aef499f ("mm/usercopy: Detect vmalloc overruns")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Tested-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220612213227.3881769-2-willy@infradead.org