f2fs: add core functions for rb-tree extent cache

This patch adds core functions including slab cache init function and
init/lookup/update/shrink/destroy function for rb-tree based extent cache.

Thank Jaegeuk Kim and Changman Lee as they gave much suggestion about detail
design and implementation of extent cache.

Todo:
 * register rb-based extent cache shrink with mm shrink interface.

v2:
 o move set_extent_info and __is_{extent,back,front}_mergeable into f2fs.h.
 o introduce __{attach,detach}_extent_node for code readability.
 o add cond_resched() when fail to invoke kmem_cache_alloc/radix_tree_insert.
 o fix some coding style and typo issues.

v3:
 o fix oops due to using an unassigned pointer.
 o use list_del to remove extent node in shrink list.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Changman Lee <cm224.lee@samsung.com>
[Jaegeuk Kim: add static for some funcitons and declare in f2fs.h]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

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