Btrfs: fs/btrfs/volumes.c: remove useless kzalloc

The call to kzalloc is followed by a kmalloc whose result is stored in the
same variable.

The semantic match that finds the problem is as follows:
(http://www.emn.fr/x-info/coccinelle/)

// <smpl>
@r exists@
local idexpression x;
statement S;
expression E;
identifier f,l;
position p1,p2;
expression *ptr != NULL;
@@

(
if ((x@p1 = \(kmalloc\|kzalloc\|kcalloc\)(...)) == NULL) S
|
x@p1 = \(kmalloc\|kzalloc\|kcalloc\)(...);
...
if (x == NULL) S
)
<... when != x
     when != if (...) { <+...x...+> }
x->f = E
...>
(
 return \(0\|<+...x...+>\|ptr\);
|
 return@p2 ...;
)

@script:python@
p1 << r.p1;
p2 << r.p2;
@@

print "* file: %s kmalloc %s return %s" % (p1[0].file,p1[0].line,p2[0].line)
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/volumes.c

@@ -2894,10 +2894,6 @@ static int read_one_chunk(struct btrfs_root *root, struct btrfs_key *key,
 		free_extent_map(em);
 	}
 
-	map = kzalloc(sizeof(*map), GFP_NOFS);
-	if (!map)
-		return -ENOMEM;
-
 	em = alloc_extent_map(GFP_NOFS);
 	if (!em)
 		return -ENOMEM;