drivers/block/ramshared: add hardware-accelerated VRAM block driver

Add the RamShared driver core in drivers/block/ramshared/ supporting direct PCIe DMA aperture mapping, blk-mq request dispatch, and synchronous .rw_page swap fast-path.

---
v1 -> v2 changes:
- queue.c: use check_shl_overflow() to prevent 64-bit integer overflow when shifting sector to byte offsets.
- queue.c: enforce PCIe BAR0 boundary validation before memory-mapped I/O.
- main.c: ensure pci_clear_master() is called during linear error unwinding in probe failure paths and device teardown.
- main.c: clamp queue_depth module parameter within [1..4096].
- ramshared.h: wrap function declarations to conform to 80-column kernel coding style.

 drivers/block/ramshared/Kconfig     | 15 +
 drivers/block/ramshared/Makefile    | 18 +
 drivers/block/ramshared/compat.h    | 105 +++++
 drivers/block/ramshared/dma.c       | 56 +++
 drivers/block/ramshared/main.c      | 181 ++++++++
 drivers/block/ramshared/queue.c     | 264 ++++++++++
 drivers/block/ramshared/ramshared.h | 63 +++
 7 files changed, 702 insertions(+)

new file mode 100644
@@ -0,0 +1,15 @@
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# RamShared block driver configuration
+#
+
+config BLK_DEV_RAMSHARED
+	tristate "RamShared Hardware-Accelerated VRAM Block Device"
+	depends on BLOCK && PCI && MMU
+	select BLK_MQ_PCI
+	help
+	  This driver provides a high-performance block device backed by
+	  discrete GPU video memory (VRAM) via direct PCIe DMA transfers.
+
+	  To compile this driver as a module, choose M here: the module
+	  will be called ramshared.

new file mode 100644
@@ -0,0 +1,18 @@
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# Makefile for the RamShared in-tree kernel driver.
+#
+
+obj-$(CONFIG_BLK_DEV_RAMSHARED) += ramshared.o
+
+ramshared-y := main.o dma.o queue.o
+
+# Standalone build support
+KDIR ?= /lib/modules/$(shell uname -r)/build
+PWD := $(shell pwd)
+
+default:
+	$(MAKE) -C $(KDIR) M=$(PWD) CONFIG_BLK_DEV_RAMSHARED=m modules
+
+clean:
+	$(MAKE) -C $(KDIR) M=$(PWD) clean

Connect drivers/block/ramshared to drivers/block/Kconfig and drivers/block/Makefile under the CONFIG_BLK_DEV_RAMSHARED symbol.

---
 drivers/block/Kconfig  | 2 ++
 drivers/block/Makefile | 1 +
 2 files changed, 3 insertions(+)

index 8e49b12..d3a5e81 100644
@@ -375,3 +375,5 @@ config BLK_DEV_ZONED_LOOP
 	  If unsure, say N.
 
+source "drivers/block/ramshared/Kconfig"
+
 endif # BLK_DEV

index 28cb489..f895c11 100644
@@ -41,3 +41,4 @@ obj- += zloop.o
 
+obj-	+= ramshared/
 
 swim_mod-y	:= swim.o swim_asm.o