mm: fix dereferencing possible ERR_PTR

Smatch checker complains that 'secretmem_mnt' dereferencing possible
ERR_PTR().  Let the function return if 'secretmem_mnt' is ERR_PTR, to
avoid deferencing it.

Link: https://lkml.kernel.org/r/20220904074647.GA64291@cloud-MacBookPro
Fixes: 1507f51255 ("mm: introduce memfd_secret system call to create "secret" memory areas")
Signed-off-by: Binyi Han <dantengknight@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foudation.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Cc: Hagen Paul Pfeifer <hagen@jauu.net>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/secretmem.c

@@ -285,7 +285,7 @@ static int secretmem_init(void)
 
 	secretmem_mnt = kern_mount(&secretmem_fs);
 	if (IS_ERR(secretmem_mnt))
-		ret = PTR_ERR(secretmem_mnt);
+		return PTR_ERR(secretmem_mnt);
 
 	/* prevent secretmem mappings from ever getting PROT_EXEC */
 	secretmem_mnt->mnt_flags |= MNT_NOEXEC;