sys/vfs/hammer: Panic with "for redo" on insufficient FIFO for redo

sys/vfs/hammer: Panic with "for redo" on insufficient FIFO for redo

hammer_generate_undo() and hammer_generate_redo() both have
the same panic message "insufficient undo FIFO space!".

Add "for redo" for hammer_generate_redo() so it's easier to
distinguish which one users are talking about when they report
these.

Since 903fdd05 hammer uses hpanic() with __func__ embedded
instead of raw panic(), but this is still considered useful
given that users don't necessary care kernel internal.