vfs: Fase 3 - filesystem type registry and superblock

vfs: Fase 3 - filesystem type registry and superblock

- Added vfs_fs_type_t structure with name, flags, and mount function
- Implemented vfs_fs_type_register and vfs_fs_type_find functions
- Registered FAT and ext2 filesystem types in init_start
- Refactored init_mount_fs to use vfs_fs_type_find instead of hardcoded strcmp
- Added vfs_superblock_t structure with fstype, bdev, lba, and private_data
- Added sb field to vfs_mount struct
- Updated vfs_mount_nolock_full to accept and store vfs_superblock_t*
- Updated all callers to pass NULL for sb parameter (virtual FS don't use it yet)