fs: support relative paths with FSCONFIG_SET_STRING

The fs_lookup_param did not consider the relative path for block device.
When we mount ext4 with journal_path option using relative path,
param->dirfd was not set which will cause mounting error.

This can be reproduced easily like this:

mke2fs -F -O journal_dev $JOURNAL_DEV -b 4096 100M
mkfs.ext4 -F -J device=$JOURNAL_DEV -b 4096 $FS_DEV
cd /dev; mount -t ext4 -o journal_path=`basename $JOURNAL_DEV` $FS_DEV $MNT

Fixes: 461c3af0 ("ext4: Change handle_mount_opt() to use fs_parameter")
Suggested-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Link: https://lore.kernel.org/r/20240925015624.3817878-1-lihongbo22@huawei.com


Signed-off-by: Christian Brauner <brauner@kernel.org>