fuse: BUG_ON correction in fuse_dev_splice_write()

fuse: BUG_ON correction in fuse_dev_splice_write()

[ Upstream commit 0e9fb6f17ad5b386b75451328975a07d7d953c6d ]

commit 963545357202 ("fuse: reduce allocation size for splice_write")
changed size of bufs array, so BUG_ON which checks the index of the array
shold also be fixed.

[SzM: turn BUG_ON into WARN_ON]

Fixes: 963545357202 ("fuse: reduce allocation size for splice_write")
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>