staging:lustre: Fix sparse warning of incorrect argument type

This patch fixes the following sparse warning :
drivers/staging/lustre/lustre/lmv/lproc_lmv.c:99:35: warning: incorrect type in argument 2 (different address spaces)
drivers/staging/lustre/lustre/lmv/lproc_lmv.c:99:35:    expected void const [noderef] <asn:1>*from
drivers/staging/lustre/lustre/lmv/lproc_lmv.c:99:35:    got char const *buffer
Signed-off-by: Himangi Saraogi <himangi774@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/lustre/lustre/lmv/lproc_lmv.c

@@ -87,8 +87,9 @@ static int lmv_placement_seq_show(struct seq_file *m, void *v)
 
 #define MAX_POLICY_STRING_SIZE 64
 
-static ssize_t lmv_placement_seq_write(struct file *file, const char *buffer,
-				   size_t count, loff_t *off)
+static ssize_t lmv_placement_seq_write(struct file *file,
+					const char __user *buffer,
+					size_t count, loff_t *off)
 {
 	struct obd_device *dev = ((struct seq_file *)file->private_data)->private;
 	char		     dummy[MAX_POLICY_STRING_SIZE + 1];