fs/9p: remove unnecessary and overrestrictive check

This eliminates a check for shared that was overrestrictive and
prevented read-only mmaps when writeback caches weren't enabled.

Cc: stable@vger.kernel.org
Fixes: 1543b4c5 ("fs/9p: remove writeback fid and fix per-file modes")
Reported-by: Robert Schwebel <r.schwebel@pengutronix.de>
Closes: https://lore.kernel.org/v9fs/ZK25XZ%2BGpR3KHIB%2F@pengutronix.deReviewed-by: Dominique Martinet <asmadeus@codewreck.org>
Reviewed-by: Christian Schoenebeck <linux_oss@crudebyte.com>
Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>

fs/9p/vfs_file.c

@@ -505,9 +505,7 @@ v9fs_file_mmap(struct file *filp, struct vm_area_struct *vma)
 	p9_debug(P9_DEBUG_MMAP, "filp :%p\n", filp);
 
 	if (!(v9ses->cache & CACHE_WRITEBACK)) {
-		p9_debug(P9_DEBUG_CACHE, "(no mmap mode)");
-		if (vma->vm_flags & VM_MAYSHARE)
-			return -ENODEV;
+		p9_debug(P9_DEBUG_CACHE, "(read-only mmap mode)");
 		invalidate_inode_pages2(filp->f_mapping);
 		return generic_file_readonly_mmap(filp, vma);
 	}