ext4: remove redundant check for encrypted file on dio write path

Currently we don't allow direct I/O on encrypted regular files, so in
such cases we return 0 early in ext4_direct_IO().  There was also an
additional BUG_ON() check in ext4_direct_IO_write(), but it can never be
hit because of the earlier check for the exact same condition in
ext4_direct_IO().  There was also no matching check on the read path,
which made the write path specific check seem very ad-hoc.

Just remove the unnecessary BUG_ON().

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: David Gstir <david@sigma-star.at>
Reviewed-by: Jan Kara <jack@suse.cz>