F2FS_BLKSIZE and 4096 are both used in f2fs_io for
IO/buffer size, but F2FS_BLKSIZE may confuse user becasue
it may be 4KB, 16KB, so use macro F2FS_DEFAULT_BLKSIZE
to unify F2FS_BLKSIZE and 4096 in f2fs_io, also adjust
"-c" parameters in mkfs man, to be consistent with commit
c35fa8cd75ac ("mkfs.f2fs: change -c option description").
Signed-off-by: Zhiguo Niu <zhiguo.niu@xxxxxxxxxx>