btrfs: use kvzalloc to allocate btrfs_fs_info

The srcu_struct in btrfs_fs_info scales in size with NR_CPUS.  On
kernels built with NR_CPUS=8192, this can result in kmalloc failures
that prevent mounting.

There is work in progress to try to resolve this for every user of
srcu_struct but using kvzalloc will work around the failures until
that is complete.

As an example with NR_CPUS=512 on x86_64: the overall size of
subvol_srcu is 3460 bytes, fs_info is 6496.
Signed-off-by: NJeff Mahoney <jeffm@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -2974,7 +2974,7 @@ static inline void free_fs_info(struct btrfs_fs_info *fs_info)
 	kfree(fs_info->super_copy);
 	kfree(fs_info->super_for_commit);
 	security_free_mnt_opts(&fs_info->security_opts);
-	kfree(fs_info);
+	kvfree(fs_info);
 }
 
 /* tree mod log functions from ctree.c */

fs/btrfs/super.c

@@ -1545,7 +1545,7 @@ static struct dentry *btrfs_mount_root(struct file_system_type *fs_type,
 	 * it for searching for existing supers, so this lets us do that and
 	 * then open_ctree will properly initialize everything later.
 	 */
-	fs_info = kzalloc(sizeof(struct btrfs_fs_info), GFP_KERNEL);
+	fs_info = kvzalloc(sizeof(struct btrfs_fs_info), GFP_KERNEL);
 	if (!fs_info) {
 		error = -ENOMEM;
 		goto error_sec_opts;