cifs: serialize all mount attempts

RHBZ: 2008434

Some servers, such as Windows2016 have a very low number of concurrent mounts that
they allow from each client.
This can be a problem if you have a more than a handful (==3 in this case)
of cifs entries in your fstab and cause a number of the mounts there to randomly fail.

Add a global mutex and use it to serialize all mount attempts.
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/fs_context.c

@@ -37,6 +37,8 @@
 #include "rfc1002pdu.h"
 #include "fs_context.h"
 
+static DEFINE_MUTEX(cifs_mount_mutex);
+
 static const match_table_t cifs_smb_version_tokens = {
 	{ Smb_1, SMB1_VERSION_STRING },
 	{ Smb_20, SMB20_VERSION_STRING},
@@ -707,10 +709,14 @@ static int smb3_get_tree_common(struct fs_context *fc)
 static int smb3_get_tree(struct fs_context *fc)
 {
 	int err = smb3_fs_context_validate(fc);
+	int ret;
 
 	if (err)
 		return err;
-	return smb3_get_tree_common(fc);
+	mutex_lock(&cifs_mount_mutex);
+	ret = smb3_get_tree_common(fc);
+	mutex_unlock(&cifs_mount_mutex);
+	return ret;
 }
 
 static void smb3_fs_context_free(struct fs_context *fc)