uprobes: (Re)add missing get_uprobe() in __find_uprobe()

commit c6bc9bd06dff ("rbtree, uprobes: Use rbtree helpers")
accidentally removed the refcount increase. Add it again.

Fixes: c6bc9bd06dff ("rbtree, uprobes: Use rbtree helpers")
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20210209150711.36778-1-svens@linux.ibm.com