f2fs: don't bother checking for encryption key in ->write_iter()

Since only an open file can be written to, and we only allow open()ing
an encrypted file when its key is available, there is no need to check
for the key again before permitting each ->write_iter().

This code was also broken in that it wouldn't actually have failed if
the key was in fact unavailable.

Change-Id: I58ccc20ee4dda8b86298d4d0c4734c36a5a33390
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: David Gstir <david@sigma-star.at>
Acked-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>