fs/v9fs: Add KERNEL build support and enable v9fs for qemu-armv7a knsh

[jasonbu]

Summary

This PR adds VirtIO 9P filesystem (v9fs) support for BUILD_KERNEL configuration and enables it on the qemu-armv7a knsh board.

Changes

1. v9fs: add va_to_pa for kernel build compatible
   Add up_addrenv_va_to_pa() conversion for buffer addresses in v9fs client read/write operations
   This is required because in kernel build mode, virtual addresses need to be converted to physical addresses for VirtIO device DMA operations
2. fs/v9fs: add support for KERNEL build, only transfer 4K once
   Limit read/write transfer size to page boundary (CONFIG_MM_PGSIZE) in kernel build
   This handles non-contiguous physical memory pages - each transfer stays within a single physical page
   Use bufptr instead of directly modifying buffer pointer for proper iteration
3. board/armv7a/knsh: enable fdt/virtiommio/virtiov9fs support
   Enable CONFIG_DEVICE_TREE=y
   Enable CONFIG_DRIVERS_VIRTIO=y and CONFIG_DRIVERS_VIRTIO_MMIO=y
   Enable CONFIG_FS_V9FS=y and CONFIG_V9FS_VIRTIO_9P=y
   Adjust RAM settings: start from 0x40100000 with size 15728640 to accommodate FDT
4. Documents/boards/qemu-armv7a: add knsh v9fs launch method
   Add documentation for launching KNSH with VirtIO 9P filesystem
   Include QEMU command with -fsdev and -device virtio-9p-device options
   Document mount command: mount -t v9fs -o trans=virtio,tag=share /share

Testing

CI-test, qemu-v7a v9fs mount and ls check if working for review hostfiles.

# Build knsh
$ ./tools/configure.sh qemu-armv7a:knsh
$ make V=1 -j`nproc`
$ make export V=1
$ cd ../apps
$ ./tools/mkimport.sh -z -x ../nuttx/nuttx-export-*.tar.gz
$ make import -j`nproc`
$ cd ../nuttx

# Run with v9fs
$ qemu-system-arm -semihosting -M virt -m 1024 -nographic \
    -fsdev local,security_model=none,id=localshared,path=. \
    -device virtio-9p-device,id=fs0,fsdev=localshared,mount_tag=share \
    -kernel ./nuttx

# Inside nsh
nsh> mount -t v9fs -o trans=virtio,tag=share /share
nsh> ls /share

test result:

ostest_main: Exiting with status 0
nsh> ls -l /share/nuttx
 -rwxrwxr-x     3367716 /share/nuttx

[jerpelea]

please split the PR into separate contributions (PR)

fs/v9fs: Add KERNEL build support
boards/qemu: enable v9fs for qemu-armv7a knsh

[acassis]

@jasonbu nice work! Thanks for adding Documentation!

[jasonbu]

please split the PR into separate contributions (PR)

fs/v9fs: Add KERNEL build support boards/qemu: enable v9fs for qemu-armv7a knsh

hi, please view #18345, as the test should work with this patch, will rebase after the #18345 merged.