fuse2fs: fix normal (non-kernel) permissions checking

fuse2fs: fix normal (non-kernel) permissions checking

Commit 9f69dfc4e275 ("fuse2fs: implement O_APPEND correctly") defined
a new flag, A_OK, to add support for testing whether the file is valid
for append operations.  This is relevant for the check_iflags_access()
function, but when are later testing operations mask against the inode
permissions, this new flag gets in the way and causes non-root users
attempting to create new inodes in a directory to fail.  Fix this by
masking off A_OK before doing these tests.

Fixes: 9f69dfc4e275 ("fuse2fs: implement O_APPEND correctly")
Signed-off-by: Theodore Ts'o <tytso@mit.edu>