JFS: performance patch

Basically, we saw a large amount of time spent in the
jfs_strfromUCS_le() function, mispredicting the branch inside the
loop, so I just added some unlikely modifiers to the if statements to
re-ordered the code.  Again, these simple changes provided > 2 % on
spec-sfs, so please consider it for inclusion.

Signed-off-by: Dave Kleikamp <shaggy@austin.ibm.com>