Since we retry reads when we discover we read from a pointer that went
stale, if a dirty pointer is erroniously stale it would cause us to loop
retrying that read forever - unless we check before issuing the read,
while the btree is still locked, when we know that a dirty pointer
should never be stale.

This patch adds that check, along with printing some helpful debug info.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>