shmem: shmem_get_partial_folio use filemap_get_entry

To avoid use of the FGP_ENTRY flag, adapt shmem_get_partial_folio() to use
filemap_get_entry() and folio_lock() instead of __filemap_get_folio(). 
Update "page" in the comments there to "folio".

Link: https://lkml.kernel.org/r/9d1aaa4-1337-fb81-6f37-74ebc96f9ef@google.com


Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>