f2fs: consider fallocated space for SEEK_DATA

f2fs: consider fallocated space for SEEK_DATA

If an amount of data are allocated though fallocate and user writes a couple of
data among the space, f2fs should return the data offset made by user when
SEEK_DATA is requested.

For example, (N: NEW_ADDR by fallocate, X: NEW_ADDR by user)
1) fallocate 0 ~ 10MB
f -> N N N N N N N N N N N N ... N

2) write 4KB at 5MB offset
f -> N N N N N X N N N N N N ... N

3) SEEK_DATA from 0 should return 5MB offset

So, this patch adds a routine to search the first dirty page to handle that.
Then, the SEEK_DATA flow skips NEW_ADDR offsets until any dirty page is found.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>