vfs: change mount API to return vfs_mount_result_t {root, sb} (P2.1)

vfs: change mount API to return vfs_mount_result_t {root, sb} (P2.1)

- Add vfs_mount_result_t structure with root and superblock
- Update vfs_fs_type_t.mount() to return vfs_mount_result_t
- Update fat_mount() and ext2_mount() to build and return superblock
- Update init_mount_fs() to handle vfs_mount_result_t and set fstype in sb
- Update vfs_mount_full() to accept sb parameter
- Update all vfs_mount_full() callers to pass sb (NULL for virtual FS)