smb: client: convert cifs_smb3_do_mount() to sget_fc()

smb: client: convert cifs_smb3_do_mount() to sget_fc()

The CIFS mount path already runs through fs_context: smb3_get_tree()
calls smb3_get_tree_common() with a struct fs_context * in hand. But
the fc is dropped on the way to sget(). Plumb it through to sget_fc()
so the legacy sget() interface can go.

cifs_smb3_do_mount() now takes (struct fs_context *, struct
smb3_fs_context *). The old (fs_type, flags) pair is reconstructed
from fc->fs_type and fc->sb_flags. The flags argument was always
passed as 0 by the sole caller anyway. The cifs_dbg diagnostic now
prints fc->sb_flags directly.

cifs_match_super() and cifs_set_super() were the two void-data
callbacks for sget(). The match callback now takes
(struct super_block *, struct fs_context *) and reads struct
cifs_mnt_data out of fc->sget_key. The set callback is gone entirely:
sget_fc() pre-populates sb->s_fs_info from fc->s_fs_info before
invoking set() so set_anon_super_fc() (which just allocates an anon
bdev) is sufficient.

Before sget_fc() we stash cifs_sb in fc->s_fs_info, the per-mount data
in fc->sget_key and force fc->sb_flags to SB_NODIRATIME | SB_NOATIME
to reproduce the previous hard-coded behaviour (alloc_super() reads
fc->sb_flags). The original sb_flags is saved and restored around the
call so the rest of the mount path sees the same fc semantics as
before.

mnt_data.flags keeps its historical value of 0 so the CIFS_MS_MASK
comparison in compare_mount_options() returns the same (always-equal)
result.

No functional change. With this in place sget() has no remaining CIFS
caller.

Link: https://patch.msgid.link/20260529-work-sget-v2-3-57bbe08604e4@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>