usb: f_fs: Clear OS Extended descriptor counts to zero in ffs_data_reset()

For userspace functions using OS Descriptors, if a function also supplies
Extended Property descriptors currently the counts and lengths stored in
the ms_os_descs_ext_prop_{count,name_len,data_len} variables are not
getting reset to 0 during an unbind or when the epfiles are closed. If
the same function is re-bound and the descriptors are re-written, this
results in those count/length variables to monotonically increase
causing the VLA allocation in _ffs_func_bind() to grow larger and larger
at each bind/unbind cycle and eventually fail to allocate.

Fix this by clearing the ms_os_descs_ext_prop count & lengths to 0 in
ffs_data_reset().

Change-Id: I3b292fe5386ab54b53df2b9f15f07430dc3df24a
Fixes: f0175ab5 ("usb: gadget: f_fs: OS descriptors support")
Cc: stable@vger.kernel.org
Signed-off-by: Udipto Goswami <ugoswami@codeaurora.org>