Discard O_APPEND flag during FUSE open/create

With write_back_cache enabled, the kernel already handles the O_APPEND
flag. Setting it on the lower fs fd can lead to file corruption with
multiple FUSE threads writing.

Test: Whatsapp backups without corruption
Bug: 160284525
Bug: 160231482
Change-Id: Iba893e905fcbf0ef99e9ebc16d3341e692bb036d
1 file changed