Btrfs: check return value of alloc_extent_map()

I add the check on the return value of alloc_extent_map() to several places.
In addition, alloc_extent_map() returns only the address or NULL.
Therefore, check by IS_ERR() is unnecessary. So, I remove IS_ERR() checking.
Signed-off-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent-tree.c

@@ -6584,7 +6584,7 @@ static noinline int relocate_data_extent(struct inode *reloc_inode,
 	u64 end = start + extent_key->offset - 1;
 
 	em = alloc_extent_map(GFP_NOFS);
-	BUG_ON(!em || IS_ERR(em));
+	BUG_ON(!em);
 
 	em->start = start;
 	em->len = extent_key->offset;

fs/btrfs/extent_map.c

@@ -51,8 +51,8 @@ struct extent_map *alloc_extent_map(gfp_t mask)
 {
 	struct extent_map *em;
 	em = kmem_cache_alloc(extent_map_cache, mask);
-	if (!em || IS_ERR(em))
-		return em;
+	if (!em)
+		return NULL;
 	em->in_tree = 0;
 	em->flags = 0;
 	em->compress_type = BTRFS_COMPRESS_NONE;

fs/btrfs/file.c

@@ -185,6 +185,7 @@ int btrfs_drop_extent_cache(struct inode *inode, u64 start, u64 end,
 			split = alloc_extent_map(GFP_NOFS);
 		if (!split2)
 			split2 = alloc_extent_map(GFP_NOFS);
+		BUG_ON(!split || !split2);
 
 		write_lock(&em_tree->lock);
 		em = lookup_extent_mapping(em_tree, start, len);

fs/btrfs/inode.c

@@ -644,6 +644,7 @@ static noinline int submit_compressed_extents(struct inode *inode,
 					async_extent->ram_size - 1, 0);
 
 		em = alloc_extent_map(GFP_NOFS);
+		BUG_ON(!em);
 		em->start = async_extent->start;
 		em->len = async_extent->ram_size;
 		em->orig_start = em->start;
@@ -820,6 +821,7 @@ static noinline int cow_file_range(struct inode *inode,
 		BUG_ON(ret);
 
 		em = alloc_extent_map(GFP_NOFS);
+		BUG_ON(!em);
 		em->start = start;
 		em->orig_start = em->start;
 		ram_size = ins.offset;
@@ -1169,6 +1171,7 @@ static noinline int run_delalloc_nocow(struct inode *inode,
 			struct extent_map_tree *em_tree;
 			em_tree = &BTRFS_I(inode)->extent_tree;
 			em = alloc_extent_map(GFP_NOFS);
+			BUG_ON(!em);
 			em->start = cur_offset;
 			em->orig_start = em->start;
 			em->len = num_bytes;