md/bitmap: call bitmap_file_unmap once bitmap_storage_alloc returns -ENOMEM

It is possible that bitmap_storage_alloc could return -ENOMEM,
and some member inside store could be allocated such as filemap.

To avoid memory leak, we need to call bitmap_file_unmap to free
those members in the bitmap_resize.

Reviewed-by: NeilBrown <neilb@suse.com>
Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>