sysfs: minimize heap allocations of sysfs paths

sysfs: minimize heap allocations of sysfs paths

11a0918a9972 ("nvme: allow to overwrite base sysfs path")
added support for changing the sysfs path via an environment variable.
Unfortunately, it added a heap allocation
every time a sysfs path was requested.

Modify the callers to not free the paths, which allows a string constant
to be returned if the path isn't overridden, avoiding an allocation.
Cache the path in a static variable so that if it is overridden,
the heap-allocated string only needs to be constructed once
and afterwards can be reused.

Create a file sysfs.c to consolidate this logic
instead of spreading them across 3 files.
Also introduce a helper to factor out the duplicated code.

Fixes: 11a0918a9972 ("nvme: allow to overwrite base sysfs path")
Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>