nilfs: fix breakage caused by barrier flag changes

nilfs: fix breakage caused by barrier flag changes

After merging the block tree, today's linux-next build (powerpc ppc64_defconfig)
failed like this:

fs/nilfs2/the_nilfs.c: In function 'nilfs_discard_segments':
fs/nilfs2/the_nilfs.c:673: error: 'DISCARD_FL_BARRIER' undeclared (first use in this function)

Caused by commit f28149616c2fab55e24c88c6ed1f89202531acd5 ("blkdev:
generalize flags for blkdev_issue_fn functions") interacting with commit
8d7a67c1003150e5a5c6985f5f76f87b09ed58de ("nilfs2: issue discard request
after cleaning segments") (which netered Linus' tree on about March 4 -
before v2.6.34-rc1).

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>