staging: lustre: mdt: fail FMODE_WRITE open if the client is read only

O_WRONLY/O_RDWR open on a file will get EROFS on a read only client,
but the rpc gets sent to the mdt anyway.
mdt will increase the mot_write_count of the mdt object, blocking
subsequent FMODE_EXEC open to the same file.

This patch makes sure we fail the FMODE_WRITE open with EROFS on the
client straight away without sending the rpc to mdt.
Signed-off-by: NLi Dongyang <dongyang.li@anu.edu.au>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-7727
Reviewed-on: http://review.whamcloud.com/18242Reviewed-by: NIan Costello <icostello@ddn.com>
Reviewed-by: NNathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: NYang Sheng <yang.sheng@intel.com>
Reviewed-by: NOleg Drokin <oleg.drokin@intel.com>
Signed-off-by: NJames Simmons <jsimmons@infradead.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/lustre/lustre/llite/namei.c

@@ -572,6 +572,10 @@ static struct dentry *ll_lookup_it(struct inode *parent, struct dentry *dentry,
 		}
 	}
 
+	if (it->it_op & IT_OPEN && it->it_flags & FMODE_WRITE &&
+	    dentry->d_sb->s_flags & MS_RDONLY)
+		return ERR_PTR(-EROFS);
+
 	if (it->it_op & IT_CREAT)
 		opc = LUSTRE_OPC_CREATE;
 	else