FUSE: Implement atomic lookup + open/create

In FUSE, as of now, uncached lookups are expensive over the wire.
E.g additional latencies and stressing (meta data) servers from
thousands of clients. These lookup calls possibly can be avoided
in some cases. Incoming three patches address this issue.


Fist patch handles the case where we are creating a file with O_CREAT.
Before we go for file creation, we do a lookup on the file which is most
likely non-existent. After this lookup is done, we again go into libfuse
to create file. Such lookups where file is most likely non-existent, can
be avoided.

Second patch handles the case where we open first time a file/dir
but do a lookup first on it. After lookup is performed we make another
call into libfuse to open the file. Now these two separate calls into
libfuse can be combined and performed as a single call into libfuse.

Here is the link to performance numbers
https://lore.kernel.org/linux-fsdevel/20220322121212.5087...


Dharmendra Singh (3):
  FUSE: Avoid lookups in fuse create
  FUSE: Rename fuse_create_open() to fuse_atomic_common()
  Implement atomic lookup + open

 fs/fuse/dir.c             | 149 +++++++++++++++++++++++++++++++++-----
 fs/fuse/fuse_i.h          |   9 +++
 include/uapi/linux/fuse.h |   5 +-
 3 files changed, 143 insertions(+), 20 deletions(-)

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v4: Addressed all comments and refactored the code into 3 patches for extended 
    create and atomic open. Dropped the patch for optimizing lookup in
    d_revalidate().
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