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

mainline inclusion
from mainline-5.12-rc1
commit b0d6d478
category: bugfix
bugzilla: https://gitee.com/src-openeuler/bpftrace/issues/I5RUM5
CVE: N/A

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b0d6d4789677d128b1933af023083054f0973574



-------------------------------------------------

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


Signed-off-by: Jason Zeng <jason.zeng@intel.com>
Signed-off-by: Aichun Shi <aichun.shi@intel.com>