vhost: check hugepage fstat error

vhost: check hugepage fstat error

Value returned from fstat is not checked for errors before being used.
This patch fixes following coverity issue.

    static uint64_t
    get_blk_size(int fd)
    {
    	struct stat stat;

    	fstat(fd, &stat);
    	return (uint64_t)stat.st_blksize;
    >>>  CID 107103 (#1 of 1): Unchecked return value from library
         (CHECKED_RETURN)
    >>>  check_return: Calling fstat(fd, &stat) without checking
         return value.
    >>>  This library function may fail and return an error code.

Fixes: 8f972312b8f4 ("vhost: support vhost-user")

Signed-off-by: Huawei Xie <huawei.xie@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>