USB: f_mtp: Improve MTP write throughput

Commit fc71db86987e8 ("USB: f_mtp: Perform vfs_write under mutex
protection) introduces mutex protection to avoid request buffer
freed while vfs_write or copy_to_user happens. This introduced
significant amount of delay for mtp_write throughput. Fix
this by reverting the patch.

Change-Id: I97e513a175b2a6691f42f8e762e2f8fd9bdf35f1
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>