bcachefs: allow passing full device path for target options

The output of mount options such as "metadata_target" in `/proc/mounts`
uses the full path to the device.

mount(8) from util-linux uses the output from `/proc/mounts` to pass
existing mount options when performing a remount, so bcachefs should
accept as input the same form that it prints as output.

Without this change:

$ mount -t bcachefs -o metadata_target=vdb /dev/vdb /mnt
$ strace mount -o remount /mnt
...
fsconfig(4, FSCONFIG_SET_STRING, "metadata_target", "/dev/vdb", 0) = -1 EINVAL (Invalid argument)
...
Signed-off-by: Thomas Bertschinger <tahbertschinger@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/super.c

@@ -2041,6 +2041,9 @@ int bch2_dev_resize(struct bch_fs *c, struct bch_dev *ca, u64 nbuckets)
 /* return with ref on ca->ref: */
 struct bch_dev *bch2_dev_lookup(struct bch_fs *c, const char *name)
 {
+	if (!strncmp(name, "/dev/", strlen("/dev/")))
+		name += strlen("/dev/");
+
 	for_each_member_device(c, ca)
 		if (!strcmp(name, ca->name))
 			return ca;