btrfs: canonicalize the device path before adding it

[PROBLEM]
Currently btrfs accepts any file path for its device, resulting some
weird situation:

 # ./mount_by_fd /dev/test/scratch1  /mnt/btrfs/

The program has the following source code:

 #include <fcntl.h>
 #include <stdio.h>
 #include <sys/mount.h>

 int main(int argc, char *argv[]) {
	int fd = open(argv[1], O_RDWR);
	char path[256];
	snprintf(path, sizeof(path), "/proc/self/fd/%d", fd);
	return mount(path, argv[2], "btrfs", 0, NULL);
 }

Then we can have the following weird device path:

 BTRFS: device fsid 2378be81-fe12-46d2-a9e8-68cf08dd98d5 devid 1 transid 7 /proc/self/fd/3 (253:2) scanned by mount_by_fd (18440)

Normally it's not a big deal, and later udev can trigger a device path
rename. But if udev didn't trigger, the device path "/proc/self/fd/3"
will show up in mtab.

[CAUSE]
For filename "/proc/self/fd/3", it means the opened file descriptor 3.
In above case, it's exactly the device we want to open, aka points to
"/dev/test/scratch1" which is another symlink pointing to "/dev/dm-2".

Inside kernel we solve the mount source using LOOKUP_FOLLOW, which
follows the symbolic link and grab the proper block device.

But inside btrfs we also save the filename into btrfs_device::name, and
utilize that member to report our mount source, which leads to the above
situation.

[FIX]
Instead of unconditionally trust the path, check if the original file
(not following the symbolic link) is inside "/dev/", if not, then
manually lookup the path to its final destination, and use that as our
device path.

This allows us to still use symbolic links, like
"/dev/mapper/test-scratch" from LVM2, which is required for fstests runs
with LVM2 setup.

And for really weird names, like the above case, we solve it to
"/dev/dm-2" instead.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Link: https://bugzilla.suse.com/show_bug.cgi?id=1230641


Reported-by: Fabian Vogt <fvogt@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>