fuse: Rearrange fuse_allow_current_process checks

fuse: Rearrange fuse_allow_current_process checks

This is a followup to a previous commit of mine [0], which added the
allow_sys_admin_access && capable(CAP_SYS_ADMIN) check.  This patch
rearranges the order of checks in fuse_allow_current_process without
changing functionality.

Commit 9ccf47b26b73 ("fuse: Add module param for CAP_SYS_ADMIN access
bypassing allow_other") added allow_sys_admin_access &&
capable(CAP_SYS_ADMIN) check to the beginning of the function, with the
reasoning that allow_sys_admin_access should be an 'escape hatch' for users
with CAP_SYS_ADMIN, allowing them to skip any subsequent checks.

However, placing this new check first results in many capable() calls when
allow_sys_admin_access is set, where another check would've also returned
1.  This can be problematic when a BPF program is tracing capable() calls.

At Meta we ran into such a scenario recently.  On a host where
allow_sys_admin_access is set but most of the FUSE access is from processes
which would pass other checks - i.e.  they don't need CAP_SYS_ADMIN 'escape
hatch' - this results in an unnecessary capable() call for each fs op.  We
also have a daemon tracing capable() with BPF and doing some data
collection, so tracing these extraneous capable() calls has the potential
to regress performance for an application doing many FUSE ops.

So rearrange the order of these checks such that CAP_SYS_ADMIN 'escape
hatch' is checked last.  Add a small helper, fuse_permissible_uidgid, to
make the logic easier to understand.  Previously, if allow_other is set on
the fuse_conn, uid/git checking doesn't happen as current_in_userns result
is returned.  These semantics are maintained here: fuse_permissible_uidgid
check only happens if allow_other is not set.

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>