usb: f_fs: Fix use-after-free for epfile

Commit 1c3b63b8 ("usb: f_fs: Avoid use-after-free of
epfile") fixed use-after-free in ffs_func_eps_disable but
there is still a possiblilty of ffs_epfile_release going
ahead and freeing the buffer while ffs_func_eps_disable has
acquired the spin_lock and initialized local copy of epfile.
Since ffs_epfile_release doesn't have spin_lock protection
for freeing up the read buffer, fix this by putting it under
spin_lock protection as well.

Change-Id: I85b1a0aea88c0033fbeef4c5db5104caac211540
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>