ANDROID: ABI break fix caused by kernfs_root and kernfs_elem_dir size increase

Adding kernfs_rwsem in kernfs_root breaks the ABI unfortunately
since it changes the size of kernfs_root structure.
To fix the issue, this patch introduces new data structure
kernfs_root_ext which includes kernfs_root with kernfs_rwsem to
avoid increasing kernfs_root'size. It also introduces kernfs_rwsem
wrapper function to reach kernfs_rwsem from kernfs_root to minimize
change.

Adding rev field into kernfs_elem_dir breaks the ABI. To fix the issue,
introduce a new kernfs_node_ext data structure which will be allocated
instead of kernfs_node and will contain the necessary field.

Bug: 320903885
Bug: 219424218
Bug: 206126556
Signed-off-by: Minchan Kim <minchan@google.com>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: Iabaad9623e9a101210073db3106c93f06847a8b3