Fixes esp8266#7426

LittleFS doesn't update the on-flash data structures when a file is
reopened as O_TRUNC until the file is closed.  This means the space of
the original, inaccessible file cannot be used, causing OOS errors in
cases when a large file is being overwritten.

Explicitly call the file sync operation to update the on-flash metadata
as soon as a file is opened.  For normal files it's a no-op, but for
O_TRUNC modes it will free the space, allowing full overwrite of large
files.