usb: gadget: f_fs: simplify ffs_dev name handling

usb: gadget: f_fs: simplify ffs_dev name handling

Currently ffs_dev::name can be either allocated by the client of
the ffs_dev structure or by the f_fs.c core itself.  The former
is used by g_ffs while the latter happens with configfs.

Historically, g_ffs did not need to allocate separate buffer for
the name so what is now f_fs.c core never cared about freeing
that space.  With configfs the name needs to be copied since the
memory is not guaranteed to be availeble after ffs_set_inst_name
finishes.

The complication is therefore here to avoid allocations in the
g_ffs case but it complicates the code inproportinally to
benefits it provides.  In particular, g_ffs is considered
‘legacy’ so optimising for its sake is unlikely to be worth the
effort.

With that observation in mind, simplify the code by unifying the
code paths in g_ffs and configfs paths.  Furthermore, instead of
allocating a new buffer for the name, simply embed it in the
ffs_dev structure.  This further makes the memory management
less convoluted and error-prone.

The configfs interface for functionfs imposed a limit of 40
characters for the name so this results in a 41-byte buffer
added to the structure.  (For short names this may lead to
wasted memory but the actual amount is not immediately obvious
and depends on pointer size and which slab buckets the structure
and name would fall into).

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>