posix: perform readdir filling in locked region

When two application threads share an open dir fd (DIR *) and issue
readdirs, storage/posix will receive separate readdir fops in separate
threads in parallel. This has two-fold issues

1. In the following pair of operations -

   entry = readdir(dir)
       and
   strcpy (gf_dirent->name, entry->d_name)

   @entry is a static buffer in libc which can get reused by another thread
   to get filled with a longer name. This can cause the second operation
   to overflow the buffer as the allocation was for the smaller name.

2. In the following pair of operations -

   seekdir (dir, offset)
      and
   entry = readdir(dir)

   If two threads are executing these sequence in parallel in separate
   threads, then one of them will end up reading wrong/unexpected entries.

It would be sufficient to fix 1. by using readdir_r but that still keeps
the second race open. Hence the patch moves all the set of operations to a
locked region which solves both races.

Signed-off-by: Anand Avati <avati@gluster.com>

BUG: 3171 (Crash in server)
URL: http://bugs.gluster.com/cgi-bin/bugzilla3/show_bug.cgi?id=3171

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