xfsprogs: fix build warnings in repair V2

xfsprogs: fix build warnings in repair V2

Rewrite the loop in btree_get_prev() so that the compiler
can see that it returns if the cur->index is zero so it
doesn't complain about possible array bound underflows
when getting the key out of the buffer. Version 2 fixes
a height overflow in the reworked loop.

Fix the directory name sign warnings by casting to (uchar_t *)
appropriately.

Signed-off-by: Dave Chinner <david@fromorbit.com>