ext4: in ext4_dir_llseek, check syscall bitness directly

ext4 treats directory offsets differently for 32-bit and 64-bit callers.
Check the caller type using in_compat_syscall, not is_compat_task.  This
changes behavior on SPARC slightly.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>