btrfs: initialize inode::file_extent_tree after i_mode has been set

btrfs_init_file_extent_tree() uses S_ISREG() to determine if the file is
a regular file. In the beginning of btrfs_read_locked_inode(), the i_mode
hasn't been read from inode item, then file_extent_tree won't be used at
all in volumes without NO_HOLES.

Fix this by calling btrfs_init_file_extent_tree() after i_mode is
initialized in btrfs_read_locked_inode().

Fixes: 3d7db6e8 ("btrfs: don't allocate file extent tree for non regular files")
CC: stable@vger.kernel.org # 6.12+
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: austinchang <austinchang@synology.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>