ANDROID: mm: unlock the page on speculative fault retry

It is observed that certain file accesses are failing when
speculative file faults are enabled via "allow_file_spec_access".
This is because of not unlocking the page on error in
filemap_map_pages, and the locked page causes endless retry of
fault.

Bug: 199706590
Fixes: 35eacb5c ("ANDROID: mm: allow vmas with vm_ops to be speculatively handled")
Change-Id: Ic7643ea8188aa281754318866fde09eea094c5da
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>