ramfs: fix mount source show for ramfs

ramfs: fix mount source show for ramfs

ramfs_parse_param does not parse key "source", and will convert
-ENOPARAM to 0. This will skip vfs_parse_fs_param_source in vfs_parse_fs_param, which
lead always "none" mount source for ramfs.

Fix it by parsing "source" in ramfs_parse_param like cgroup1_parse_param
does.

Link: https://lkml.kernel.org/r/20210924091756.1906118-1-yangerkun@huawei.com
Signed-off-by: yangerkun <yangerkun@huawei.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>