hfsplus: introduce alloc_mutex

Use a new per-sb alloc_mutex instead of abusing i_mutex of the alloc_file
to protect block allocations.  This gets rid of lockdep nesting warnings
and prepares for extending the scope of alloc_mutex.

Signed-off-by: Christoph Hellwig <hch@tuxera.com>
diff --git a/fs/hfsplus/hfsplus_fs.h b/fs/hfsplus/hfsplus_fs.h
index dc856be..df0a631 100644
--- a/fs/hfsplus/hfsplus_fs.h
+++ b/fs/hfsplus/hfsplus_fs.h
@@ -116,6 +116,9 @@
 	struct inode *hidden_dir;
 	struct nls_table *nls;
 
+	/* synchronize block allocations */
+	struct mutex alloc_mutex;
+
 	/* Runtime variables */
 	u32 blockoffset;
 	u32 sect_count;