staging: erofs: fix to handle error path of erofs_vmap()

erofs_vmap() wrapped vmap() and vm_map_ram() to return virtual
continuous memory, but both of them can failed due to a lot of
reason, previously, erofs_vmap()'s callers didn't handle them,
which can potentially cause NULL pointer access, fix it.

Fixes: 3883a79a ("staging: erofs: introduce VLE decompression support")
Fixes: 0d40d6e3 ("staging: erofs: add a generic z_erofs VLE decompressor")
Cc: <stable@vger.kernel.org> # 4.19+
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>