xfs: remove __KERNEL__ check from xfs_dir2_leaf.c

It's actually an ifndef section, which means it is only included in
userspace. however, it's deep within the libxfs code, so it's
unlikely that the condition checked in userspace can actually occur
(search an empty leaf) through the libxfs interfaces. i.e. if it can
happen in usrspace, it can happen in the kernel, so remove it from
userspace too....

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>