Revert fwalk/sfp locking to fix concurrent reads

The locking can fail in a couple of ways:

 - A concurrent fread from an unbuffered or line-buffered file flushes
   the output of other line-buffered files, and if _fwalk locks every
   file, then the fread blocks until other file reads have completed.

 - __sfp can initialize a file lock while _fwalk is locking/unlocking it.

For now, revert to the behavior Bionic had in previous releases. This
commit reverts the file locking parts of commit
468efc80.

Bug: http://b/131251441
Bug: http://b/130189834
Test: bionic unit tests
Change-Id: I9e20b9cd8ccd14e7962f7308e174f08af72b56c6
(cherry picked from commit c485cdb0249415b8aee5968b2b8854921e152854)