proc: convert smaps_pmd_entry to use a folio

Replace two calls to compound_head() with one.

Link: https://lkml.kernel.org/r/20240403171456.1445117-5-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Christian Brauner <brauner@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>