usb: f_fs: Prevent race & use after free scenario on epfile

In case of fast composition switch along with disconnect/connect
performed, there are chances that the process of ep_disable,
function_disable, epfile_release & ep0_release race between each
other causing use after free scenarios.
This is seen with epfile struct itself and also with the read_buffer
member variable as well.

This change is a squash of below commits.

(1c3b63b8d818) usb: f_fs: Avoid use-after-free of epfile.
(971a85773340) usb: f_fs: Fix use-after-free for epfile.
(a008a43c5cd8) usb: f_fs: Prevent race between ep0_release & reset_work.
(f7fbba49e4a8) usb: f_fs: Fix Double free from ffs_data_clear.

Change-Id: If3f69e1e25be810aa82931b65fa2506b2e9f33bd
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>
Signed-off-by: Udipto Goswami <quic_ugoswami@quicinc.com>