NFS: Squelch compiler warning in nfs_add_server_stats()

Clean up:

fs/nfs/iostat.h: In function ‘nfs_add_server_stats’:
fs/nfs/iostat.h:41: warning: comparison between signed and unsigned integer expressions
fs/nfs/iostat.h:41: warning: comparison between signed and unsigned integer expressions
fs/nfs/iostat.h:41: warning: comparison between signed and unsigned integer expressions
fs/nfs/iostat.h:41: warning: comparison between signed and unsigned integer expressions

Commit fce22848 replaced the open-coded per-cpu logic in several
functions in fs/nfs/iostat.h with a single invocation of
this_cpu_ptr().  This macro assumes its second argument is signed,
not unsigned.
Signed-off-by: NChuck Lever <chuck.lever@oracle.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/iostat.h

@@ -36,14 +36,14 @@ static inline void nfs_inc_stats(const struct inode *inode,
 
 static inline void nfs_add_server_stats(const struct nfs_server *server,
 					enum nfs_stat_bytecounters stat,
-					unsigned long addend)
+					long addend)
 {
 	this_cpu_add(server->io_stats->bytes[stat], addend);
 }
 
 static inline void nfs_add_stats(const struct inode *inode,
 				 enum nfs_stat_bytecounters stat,
-				 unsigned long addend)
+				 long addend)
 {
 	nfs_add_server_stats(NFS_SERVER(inode), stat, addend);
 }
@@ -51,7 +51,7 @@ static inline void nfs_add_stats(const struct inode *inode,
 #ifdef CONFIG_NFS_FSCACHE
 static inline void nfs_add_fscache_stats(struct inode *inode,
 					 enum nfs_stat_fscachecounters stat,
-					 unsigned long addend)
+					 long addend)
 {
 	this_cpu_add(NFS_SERVER(inode)->io_stats->fscache[stat], addend);
 }