dax: skip read lock assertion for read-only filesystems

The commit 168316db("dax: assert that i_rwsem is held
exclusive for writes") added lock assertions to ensure proper
locking in DAX operations. However, these assertions trigger
false-positive lockdep warnings since read lock is unnecessary
on read-only filesystems(e.g., erofs).

This patch skips the read lock assertion for read-only filesystems,
eliminating the spurious warnings while maintaining the integrity
checks for writable filesystems.

Fixes: 168316db ("dax: assert that i_rwsem is held exclusive for writes")
Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Reviewed-by: Friendy Su <friendy.su@sony.com>
Reviewed-by: Daniel Palmer <daniel.palmer@sony.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>