In order to constify other functions, we need page_ext_get() to be const. 
This is no problem as lookup_page_ext() already takes a const argument.

Link: https://lkml.kernel.org/r/20240326171045.410737-4-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>