FS: JFS: Check for read-only mounted filesystem in txBegin

[ Upstream commit 95e2b352 ]

 This patch adds a check for read-only mounted filesystem
 in txBegin before starting a transaction potentially saving
 from NULL pointer deref.

Signed-off-by: Immad Mir <mirimmad17@gmail.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>