Assert for Direct IO at the beginning in PositionedRead (#3891)

Summary:
Moved the direct-IO assertion to the top in `PosixSequentialFile::PositionedRead`, as it doesn't make sense to check for sector alignments before checking for direct IO.
Closes https://github.com/facebook/rocksdb/pull/3891

Differential Revision: D8267972

Pulled By: sagar0

fbshipit-source-id: 0ecf77c0fb5c35747a4ddbc15e278918c0849af7

env/io_posix.cc

@@ -184,16 +184,15 @@ Status PosixSequentialFile::Read(size_t n, Slice* result, char* scratch) {
 
 Status PosixSequentialFile::PositionedRead(uint64_t offset, size_t n,
                                            Slice* result, char* scratch) {
-  if (use_direct_io()) {
+  assert(use_direct_io());
   assert(IsSectorAligned(offset, GetRequiredBufferAlignment()));
   assert(IsSectorAligned(n, GetRequiredBufferAlignment()));
   assert(IsSectorAligned(scratch, GetRequiredBufferAlignment()));
-  }
+
   Status s;
   ssize_t r = -1;
   size_t left = n;
   char* ptr = scratch;
-  assert(use_direct_io());
   while (left > 0) {
     r = pread(fd_, ptr, left, static_cast<off_t>(offset));
     if (r <= 0) {