reiserfs: Expand i_mutex to enclose lookup_one_len

 2.6.30-rc3 introduced some sanity checks in the VFS code to avoid NFS
 bugs by ensuring that lookup_one_len is always called under i_mutex.

 This patch expands the i_mutex locking to enclose lookup_one_len. This was
 always required, but not not enforced in the reiserfs code since it
 does locking around the xattr interactions with the xattr_sem.

 This is obvious enough, and it survived an overnight 50 thread ACL test.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>