Speed up O_APPEND with remote revalidating

Remote revalidating requires to update file size on every write
on a file opened with O_APPEND. If host FD exists, it can be
used to update the size and skip round trip to the gofer. With
this change, O_APPEND writes with remote revalidating is almost
as fast as exclusive mode:

              BM_Append
VFS1           60.7us
VFS2           56.8us
VFS2 exclusive 14.2us
This change    15.8us

Updates #1792

PiperOrigin-RevId: 369486801

0 files changed, 0 insertions, 0 deletions