slub: Fix a documentation build error for krealloc()

The kerneldoc comment for krealloc() contains an unmarked literal block,
leading to these warnings in the docs build:

./mm/slub.c:4936: WARNING: Block quote ends without a blank line; unexpected unindent. [docutils]
./mm/slub.c:4936: ERROR: Undefined substitution referenced: "--------". [docutils]

Mark up and indent the block properly to bring a bit of peace to our build
logs.

Fixes: 489a744e5fb1 (mm: krealloc: clarify valid usage of __GFP_ZERO)
Signed-off-by: Jonathan Corbet <[e-mail]>
Signed-off-by: Matthew Wilcox (Oracle) <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Vlastimil Babka <[e-mail]>

mm/slub.c

···
  * When slub_debug_orig_size() is off, krealloc() only knows about the bucket
  * size of an allocation (but not the exact size it was allocated with) and
  * hence implements the following semantics for shrinking and growing buffers
- * with __GFP_ZERO.
+ * with __GFP_ZERO::
  *
- *         new             bucket
- * 0       size             size
- * |--------|----------------|
- * |  keep  |      zero      |
+ *           new             bucket
+ *   0       size             size
+ *   |--------|----------------|
+ *   |  keep  |      zero      |
  *
  * Otherwise, the original allocation size 'orig_size' could be used to
  * precisely clear the requested size, and the new size will also be stored