libjio: Fix a memory leak in jtrans_rollback()

libjio: Fix a memory leak in jtrans_rollback()

jtrans_rollback() creates a new transaction using the data saved by
jtrans_commit() in op->pdata as the data to commit, which is stored in
op->buf, making both point to the same location.

After applying the new transaction, we need to free it; however, jtrans_free()
assumes op->buf points to a different place than op->pdata, and attempts to
free both. Thus, to prevent a double free in the hand-crafted transaction, we
need to set one of them to NULL.

However, we currently set both to NULL, effectively leaking the memory used to
store it.

This patch fixes that bug, by simply removing one of the two assignments.

Signed-off-by: Alberto Bertogli <albertito@blitiri.com.ar>