Speed up search for next valid object in segment in --repair mode

When an object is corrupted, the start position of the next object
will not be known as the size field belonging to the corrupted
object may be corrupted as well. In order to find the next object
within the segment, the remainder is scanned for the next valid
object, byte-by-byte. An object is considered valid if the CRC
checksum matches the content. However, doing so the scan accepted
any object size that fit within the remainder of the segment. As a
result, in particular when the corruption occurred near the start
of a segment, CRC checksums were calculated for large objects,
often hundreds of megabytes in size, despite the size being limited
to 20 MiB. This change makes it so that CRC calculation is skipped
when the object header indicates an impossible size, thereby,
greatly reducing the number of CPU cycles used for CRC calculations.
In my case, this brought down the time for repair from hours to mere
minutes.

This has also the additional benefit that there is some verification
in addition to the CRC checksum. The 4-bytes checksum is rather
short considering the amount of data that might be in an archive.

Likely fixes the hanging --repair in #5995 also.

src/borg/repository.py

@@ -1470,10 +1470,8 @@ class LoggedIO:
                         dst_fd.write(MAGIC)
                         while len(d) >= self.header_fmt.size:
                             crc, size, tag = self.header_fmt.unpack(d[:self.header_fmt.size])
-                            if size < self.header_fmt.size or size > len(d):
-                                d = d[1:]
-                                continue
-                            if crc32(d[4:size]) & 0xffffffff != crc:
+                            if size > MAX_OBJECT_SIZE or size < self.header_fmt.size or size > len(d)
+                               or crc32(d[4:size]) & 0xffffffff != crc:
                                 d = d[1:]
                                 continue
                             dst_fd.write(d[:size])