gfs2: avoid inefficient use of crc32_le_shift()

__get_log_header() was using crc32_le_shift() to update a CRC with four
zero bytes.  However, this is about 5x slower than just CRC'ing four
zero bytes in the normal way.  Just do that instead.

(We could instead make crc32_le_shift() faster on short lengths.  But
all its callers do just fine without it, so I'd like to just remove it.)

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>