f2fs: updated to upstream branch

Squashed the following commmits:

f2fs: introduce -o fastboot for reducing booting time
f2fs: remove unnecessary macro
f2fs: avoid race condition in handling wait_io
f2fs: send discard commands in larger extent
f2fs: revisit inline_data to avoid data races and potential bugs
f2fs: remove pointless bit testing in f2fs_delete_entry()	J
f2fs: do not discard data protected by the previous checkpoint
f2fs: flush_dcache_page for inline data	Jaegeuk Kim	1	-
f2fs: call write_checkpoint under disabled gc