libfs: Add a lock class for the offset map's xa_lock

Tie the dynamically-allocated xarray locks into a single class so
contention on the directory offset xarrays can be observed.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Message-Id: <169020933088.160441.9405180953116076087.stgit@manet.1015granger.net>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/libfs.c

@@ -249,6 +249,8 @@ static u32 dentry2offset(struct dentry *dentry)
 	return (u32)((uintptr_t)(dentry->d_fsdata));
 }
 
+static struct lock_class_key simple_offset_xa_lock;
+
 /**
  * simple_offset_init - initialize an offset_ctx
  * @octx: directory offset map to be initialized
@@ -257,6 +259,7 @@ static u32 dentry2offset(struct dentry *dentry)
 void simple_offset_init(struct offset_ctx *octx)
 {
 	xa_init_flags(&octx->xa, XA_FLAGS_ALLOC1);
+	lockdep_set_class(&octx->xa.xa_lock, &simple_offset_xa_lock);
 
 	/* 0 is '.', 1 is '..', so always start with offset 2 */
 	octx->next_offset = 2;