ext4: add support for FS_IOC_GETFSSYSFSPATH

ext4: add support for FS_IOC_GETFSSYSFSPATH

The new sysfs path ioctl lets us get the /sys/fs/ path for a given
filesystem in a fs agnostic way, potentially nudging us towards
standarizing some of our reporting.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Cc: linux-ext4@vger.kernel.org
Link: https://lore.kernel.org/r/20240315035308.3563511-4-kent.overstreet@linux.dev
Signed-off-by: Theodore Ts'o <tytso@mit.edu>