NFS:remove redundant call to nfs_do_access

In function nfs_permission:
1. the rcu_read_lock and rcu_read_unlock around nfs_do_access
is unnecessary because the rcu critical data structure is already
protected in subsidiary function nfs_access_get_cached_rcu. No other
data structure needs rcu_read_lock in nfs_do_access.

2. call nfs_do_access once is enough, because:
2-1. when mask has MAY_NOT_BLOCK bit
The second call to nfs_do_access will not happen.

2-2. when mask has no MAY_NOT_BLOCK bit
The second call to nfs_do_access will happen if res == -ECHILD, which
means the first nfs_do_access goes out after statement if (!may_block).
The second call to nfs_do_access will go through this procedure once
again except continue the work after if (!may_block).
But above work can be performed by only one call to nfs_do_access
without mangling the mask flag.

Tested in x86_64
Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/dir.c

@@ -2759,14 +2759,7 @@ int nfs_permission(struct inode *inode, int mask)
 	if (!NFS_PROTO(inode)->access)
 		goto out_notsup;
 
-	/* Always try fast lookups first */
-	rcu_read_lock();
-	res = nfs_do_access(inode, cred, mask|MAY_NOT_BLOCK);
-	rcu_read_unlock();
-	if (res == -ECHILD && !(mask & MAY_NOT_BLOCK)) {
-		/* Fast lookup failed, try the slow way */
-		res = nfs_do_access(inode, cred, mask);
-	}
+	res = nfs_do_access(inode, cred, mask);
 out:
 	if (!res && (mask & MAY_EXEC))
 		res = nfs_execute_ok(inode, mask);