slub: Fix a documentation build error for krealloc()

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 <corbet@lwn.net>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://patch.msgid.link/20250611155916.2579160-6-willy@infradead.org
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>