btrfs-progs: extent-cache: actually cache extent buffers

btrfs-progs: extent-cache: actually cache extent buffers

We have the infrastructure to cache extent buffers but we don't actually
do the caching.  As soon as the last reference is dropped, the buffer
is dropped.  This patch keeps the extent buffers around until the max
cache size is reached (defaults to 25% of memory) and then it drops
the last 10% of the LRU to free up cache space for reallocation.  The
cache size is configurable (for use by e.g. lowmem) when the cache is
initialized.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
[ update codingstyle, switch total_memory to bytes ]
Signed-off-by: David Sterba <dsterba@suse.com>