shmem: document how to "persist" data when using shmem_*file_setup

Add a blurb that simply dirtying the folio will persist data for in-kernel
shmem files.  This is what most of the callers already do.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>

mm/shmem.c

@@ -2147,6 +2147,10 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
  * Looks up the page cache entry at @inode & @index.  If a folio is
  * present, it is returned locked with an increased refcount.
  *
+ * If the caller modifies data in the folio, it must call folio_mark_dirty()
+ * before unlocking the folio to ensure that the folio is not reclaimed.
+ * There is no need to reserve space before calling folio_mark_dirty().
+ *
  * When no folio is found, the behavior depends on @sgp:
  *  - for SGP_READ, *foliop is %NULL and 0 is returned
  *  - for SGP_NOALLOC, *foliop is %NULL and -ENOENT is returned