erofs: silence warnings related to impossible m_plen

Dan reported two smatch warnings [1],
.. warn: should '1 << lclusterbits' be a 64 bit type?
.. warn: should 'm->compressedlcs << lclusterbits' be a 64 bit type?

In practice, m_plen cannot be more than 1MiB due to on-disk constraint
for the compression mode, so we're always safe here.

In order to make static analyzers happy and not report again, let's
silence them instead.

[1] https://lore.kernel.org/r/202203091002.lJVzsX6e-lkp@intel.comReported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20220310173448.19962-1-hsiangkao@linux.alibaba.comSigned-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

fs/erofs/zmap.c

@@ -494,7 +494,7 @@ static int z_erofs_get_extent_compressedlen(struct z_erofs_maprecorder *m,
 	     !(vi->z_advise & Z_EROFS_ADVISE_BIG_PCLUSTER_1)) ||
 	    ((m->headtype == Z_EROFS_VLE_CLUSTER_TYPE_HEAD2) &&
 	     !(vi->z_advise & Z_EROFS_ADVISE_BIG_PCLUSTER_2))) {
-		map->m_plen = 1 << lclusterbits;
+		map->m_plen = 1ULL << lclusterbits;
 		return 0;
 	}
 	lcn = m->lcn + 1;
@@ -540,7 +540,7 @@ static int z_erofs_get_extent_compressedlen(struct z_erofs_maprecorder *m,
 		return -EFSCORRUPTED;
 	}
 out:
-	map->m_plen = m->compressedlcs << lclusterbits;
+	map->m_plen = (u64)m->compressedlcs << lclusterbits;
 	return 0;
 err_bonus_cblkcnt:
 	erofs_err(m->inode->i_sb,