pkg/report: support optional CRC in backtrace lines

At some point kmemleak started adding a CRC checksum to the "backtrace:" line in memory leak reports. The existing
regular expression did not account for this, causing parsing to fail for these reports.

Update the regex to make the CRC component optional, allowing reports both with and without the checksum to be parsed
correctly.

Fixes #4728.

Author: pimyn-girgis

pkg/report/linux.go

@@ -1148,7 +1148,7 @@ var linuxStackParams = &stackParams{
 		regexp.MustCompile(`Freed:`),
 		regexp.MustCompile(`Freed by task [0-9]+:`),
 		// Match 'backtrace:', but exclude 'stack backtrace:'
-		regexp.MustCompile(`[^k] backtrace:`),
+		regexp.MustCompile(`[^k] backtrace(?: \(crc [[:xdigit:]]*\))?:`),
 		regexp.MustCompile(`Backtrace:`),
 		regexp.MustCompile(`Uninit was stored to memory at`),
 	},
@@ -1750,7 +1750,7 @@ var linuxOopses = append([]*oops{
 				fmt:   "memory leak in %[1]v",
 				stack: &stackFmt{
 					parts: []*regexp.Regexp{
-						compile("backtrace:"),
+						compile("backtrace(?: \\(crc [[:xdigit:]]*\\))?:"),
 						parseStackTrace,
 					},
 					skip: []string{"kmemleak", "mmap", "kmem", "slab", "alloc", "create_object",