isuper-intel.c: fix double free in load_imsm_mpb()

isuper-intel.c: fix double free in load_imsm_mpb()

In load_imsm_mpb() there is potential double free issue on super->buf.

The first location to free super->buf is from get_super_block() <==
load_and_parse_mpb() <== load_imsm_mpb():
 4514         if (posix_memalign(&super->migr_rec_buf, MAX_SECTOR_SIZE,
 4515             MIGR_REC_BUF_SECTORS*MAX_SECTOR_SIZE) != 0) {
 4516                 pr_err("could not allocate migr_rec buffer\n");
 4517                 free(super->buf);
 4518                 return 2;
 4519         }

If the above error condition happens, super->buf is freed and value 2
is returned to get_super_block() eventually. Then in the following code
block inside load_imsm_mpb(),
 5289  error:
 5290         if (!err) {
 5291                 s->next = *super_list;
 5292                 *super_list = s;
 5293         } else {
 5294                 if (s)
 5295                         free_imsm(s);
 5296                 close_fd(&dfd);
 5297         }
at line 5295 when free_imsm() is called, super->buf is freed again from
the call chain free_imsm() <== __free_imsm(), in following code block,
 4651         if (super->buf) {
 4652                 free(super->buf);
 4653                 super->buf = NULL;
 4654         }

This patch sets super->buf as NULL after line 4517 in load_imsm_mpb()
to avoid the potential double free().

(Coly Li helps to re-compose the commit log)

Signed-off-by: Wu Guanghao <wuguanghao3@huawei.com>
Reviewed-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>
Acked-by: Coly Li <colyli@suse.de>
Signed-off-by: Jes Sorensen <jes@trained-monkey.org>