platform: msm: pft: fix close all encrypted files

filp_close() eventually calls pft_file_close().
Simplify pft_close_opened_enc_files() and remove having mutex
in both places that cause a deadlock.

Change-Id: I0afa2d2f0806f18d430c027601fb9974855ef185
Signed-off-by: Amir Samuelov <amirs@codeaurora.org>